0.289.0

Likely to cause new Flow errors:

For a poorly bounded generic indexed access types, we will no longer generate an error for it when it's being first subtyped against. Instead, we will make it opaque so that when it's being subtyped against, it will error. example
Flow will error when you are trying to redefine the following reserved types: $NonMaybeType, NonNullable, $ReadOnly, Readonly, $Keys, $Values, Values, $ReadOnlyMap, ReadonlyMap, $ReadOnlySet, ReadonlySet.

Notable bug fixes:

We no longer incorrectly report conditional hook call for loops with a break. example
Fixed a potential crash when evaluating a conditional type. example
Flow will now allow computed keys of type StringPrefix/StringSuffix to be included in object initializers with other non-overlapping keys (example try-Flow)

Library Definitions:

Update React API types from recent releases Added

act: 19.0: https://react.dev/reference/react/act
captureOwnerStack: 19.1: See https://react.dev/reference/react/captureOwnerStack
useEffectEvent: 19.2: https://react.dev/reference/react/useEffectEvent
<Activity />: 19.2: See https://react.dev/reference/react/Activity Updated
useDeferredValue: 19.0: Accepts a second argument for initial value
useTransition: 19.0: Accepts an async action function
startTransition: 19.0: Accepts an async action function

0.288.0

Likely to cause new Flow errors:

React.ElementProps is removed. You should use React.ElementConfig instead.
For a function named as a hook but not annotated as a hook, it will no longer be treated like a hook when you try to use it from property access or property destructuring in component syntax components and hook syntax hooks. They will now cause react-rule-hook-non-hook-syntax errors. To fix, annotate them with hook annotation. example
Calling functions passed to function and component with hook like name but not annotated as hook will now error. example.
React$RefSetter<...> and and React$Context are removed. It continues to be available in the form of React.RefSetter<...> and React.Context<...>, regardless of whether you have imported React.
React$ElementRef<...> was removed. It continues to be available in the form of React.ElementRef<...>, regardless of whether you have imported React.

Breaking Changes:

The config option max_literal_length was removed. Flow will always decide whether to infer a specific or general type of string literal based on the natural inference algorithm.

Notable bug fixes:

You can now use @@dispose and @@asyncDispose in your library typings. You can access the respective properties with obj[Symbol.dispose] and obj[Symbol.asyncDispose].
Allow comparisons of bigint singletons

Library Definitions:

The second type argument of ExactReactElement will be populated by React.ElementConfig<C> instead of React.ElementProps<C>. Most of the code that doesn't depend on exact react element types won't be affected.

0.287.0

Breaking Changes:

We will stop publishing new versions of flow-parser-bin. If you want to parse flow code with native-like performance, you should use hermes-parser package, which is running on wasm.

Likely to cause new Flow errors:

The deprecated React$Element type is now removed. You should use React.Node or React.MixedElement in most cases. If you really want exact react elements, you can use ExactReactElement_DEPRECATED

New Features:

Flow has full React 19 ref-as-prop support. Since this version, you are allowed to spread ref prop in component syntax components.

IDE:

We now provide a rewrite refactor to convert arrays, map, set, and object types into their readonly forms.

0.286.0

Breaking Changes:

Starting from this version, flow-bin will only contain ARM64 binaries for macOS.

Likely to cause new Flow errors:

Flow now errors on constant conditions and invalid comparisons. example (D83014262 samzhou19815)
Flow will no longer infer an unsound type for array and object literals passed to a general function. example
React.ElementConfig<...> and React.PropsOf<...> will now always include the ref prop in the props object type.
The config option for an earlier mode react.ref_as_prop=partial_support is removed, and Flow now has better support for React 19's ref-as-prop behavior:
1. All the ref prop in component syntax components must be made optional
2. You might need to rewrite some of your higher order components. e.g. previously an identity HOC must be written as

declare function id<Props: {...}, Ref>(c: component(ref?: Ref, ...Props)): component(ref?: Ref...Props)

now it must be written as

declare function id<Props: {...}>(c: component(...Props)): component(...Props)

0.285.0

Library Definitions:

Fix type of useActionState hook in React. Type should be Awaited instead of Promise (the opposite operation).
Added types for Object.groupBy and Map.groupBy
- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/groupBy
- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/groupBy

Bug fixes:

Fixed a bug that when experimental.ts_utility_syntax=true, imported Readonly, ReadonlyArray, NonNullable, ReadonlyMap, ReadonlySet types are considered any.

0.284.0

Likely to cause new Flow errors:

Flow now strictly enforces that Array.includes, .indexOf, and .lastIndexOf take only values that could potentially be present in the array -- it is now an error to check if arrays include values whose types are unrelated to the array's element type. This catches many cases where developers accidentally test the wrong thing, which may lead to errors at runtime.
Flow will infer general types for react jsx elements. The new behavior is described in the docs.

0.283.0

Likely to cause new Flow errors:

Invalid suppressions like // $FlowFixMe[incompatible will now cause an error on the suppression

0.282.0

Likely to cause new Flow errors:

Multiple object property invariant subtyping errors on the same object will be combined into one error. Some error locations might be moved.

Notable bug fixes:

Fixes crash happening on exported functions with signatures of the form <X>(x: X, y: typeof x): T

IDE:

We now provide a best effort quickfix for some invariant subtyping errors.

Library Definitions:

Array.find can now return a more precise value if the predicate is inferred to be a type guard. example

0.281.0

Breaking Changes:

Support for the casting_syntax=colon is removed. This means that the as casting syntax is always enabled.
experimental.error_code_migration config option is removed. In the previously version, the only allowed value was new.

Likely to cause new Flow errors:

We have fixed more source of object literal unsoundness. More errors will be exposed. (example)
For most of the common invariant subtyping errors (arrays, objects with mutable fields), we will now emit a single subtyping error with explanation how to fix. Error locations might change. See https://flow.org/en/docs/lang/variance/#toc-invariance to learn more about variance of types
Suppressions without error code will now be an error instead of a warning.
Suppressions without error code will no longer apply. You can download and use this script to automatically migrate most of the suppressions.
Only $FlowFixMe and $FlowExpectedError are supported suppression comments. Existing $FlowIgnore and $FlowIssue will no longer suppress anything.

New Features:

Added a Flow lint require-explicit-enum-checks which works like require-explicit-enum-switch-cases but for match.

Notable bug fixes:

Various codemods will now insert parentheses around expressions in class extends if they are changed. Previously, it might result in invalid code like class Foo extends myExpr() as Bar
Fixes an inconsistency in recording the this-type guard on a regular (non-declare) method in the same vs different files.

IDE:

Added a quickfix to convert type to value imports on type-as-value errors.

0.280.0

Likely to cause new Flow errors:

Accessing missing exports on namespaced import will now trigger missing-export error instead of prop-missing error. (example)
The only supported suppress_type $FlowFixMe is now just a type alias of any. For most of the code, there will be no functional differences. However, you might see new errors if you have any local definitions of $FlowFixMe, or you used the undocumented $FlowFixMe<arbitrary type arguments>.
Support for suppress_type config has been removed. The only supported supress_type now is $FlowFixMe. If you want other variants, you can add

type MySuppressType = any

in your global library definitions.

Many subtyping type errors' error codes have been standardized into incompatible-type, so some previously suppressed errors will reappear until you change the suppression error code into incompatible-type. The change was announced in the previous version, with option to enable it via experimental.error_code_migration=new. Now the only valid option to experimental.error_code_migration is new. You can run flow codemod error-code-migration --write . with the previous version of Flow to help migrate, since the codemod is removed in this version.

0.279.0

New Features:

In the next release (0.280.0) of Flow, we intend to standardize the error code for various subtyping errors into incompatible-type. You can add experimental.error_code_migration=new in your flowconfig to enable the new behavior now. We also provide a codemod flow codemod error-code-migration --write . that you can run over your codebase to automatically change the error code. Both the flowconfig option and the codemod will be removed in the next version.

Notable bug fixes:

Improved precision of error messages when inferred primitive types are checked against other incompatible primitive types (e.g. try-Flow)

Misc:

Thanks @JamBalaya56562 for fixing various typos across the codebase!

0.278.0

Likely to cause new Flow errors:

Hooks calls inside normal functions in component or hooks as conditional calls. They will get react-rule-hook-conditional error instead of react-rule-hook error.
Array literals that cannot be contextually typed will be inferred as an actual array type. It might cause additional errors. example

0.277.1

Notable bug fixes:

Fixed windows builds.

0.277.0

Likely to cause new Flow errors:

Array literals that cannot be contextually typed can no longer be passed to mutable tuples with a more general element type. It might cause additional errors. example

0.276.0

Likely to cause new Flow errors:

Hook calls inside anonymous functions bound to a variable will get react-rule-hook-definitely-not-in-component-or-hook error, if the variable name doesn't conform to hook naming convention. example

IDE:

Added support for workspace symbol feature

Misc:

Thanks @jbroma for improving as casts with function generics and component generics!

0.275.0

Likely to cause new Flow errors:

For all object literals in positions that cannot be contextually typed, we will infer a stricter type for them. It will cause new errors in code like

const foo = {baz: new Dog()};
type Foo = {bar?: string, baz: Animal};
declare function acceptFoo(foo: Foo): void;
acceptFoo(foo); // error

To fix the error, you can either annotate the object

const foo: Foo = {baz: new Dog()};

type Foo = {bar?: string, baz: Animal};
declare function acceptFoo(foo: Foo): void;

acceptFoo(foo);

or make the call site accepts readonly objects:

const foo = {baz: new Dog()};

type Foo = $ReadOnly<{bar?: string, baz: Animal}>;
declare function acceptFoo(foo: Foo): void;

acceptFoo(foo);

We provide a codemod to automate the annotation process. flow codemod annotate-literal-declaration --write --max-type-size 5. (You can adjust the max type size based on your needs).

IDE:

Support rename on private properties and methods.

Library Definitions:

React$MixedElement is removed from builtin libdef. It will cause internal-type error since v0.258.0. You should use React.MixedElement instead.

0.274.2

Bug fixes for match

0.274.1

New Features:

Support for experimental match feature using option experimental.pattern_matching=true

0.274.0

Likely to cause new Flow errors:

Unannotated object literals reachable from exports will now be inferred to have all mutable fields when being imported. Previously, it has unsound types, so new errors might appear.
When comparing two object types whose properties have variance incompatibilities, Flow will raise a single error that will summarize the properties with incompatible variances, instead of a single error for each property. (e.g. try-Flow)
When an object with extra properties is passed to a place that expect an exact object, we will now generate a single error with all extra properties. The error message will list the extra properties, and state that "Exact objects do not accept extra props". In rare cases, the error locations might be moved.
Flow will error more consistently with sketchy-bool on nullable boolean types (e.g. try-Flow)

Library Definitions:

All properties in the builtin PropertyDescriptor type are marked as readonly. If you need a mutable version, you can introduce something like type MutablePropertyDescriptor<T> = {...$Exact<PropertyDescriptor<T>>, ...}

0.273.1

Notable bug fixes:

Fixed windows builds.
Fixed crash when loading saved state.

0.273.0

Likely to cause new Flow errors:

We are announcing Natural Inference for Flow, an improved way to infer types for primitive values, that resolves a long-standing correctness gap and source of confusion. See more in this post.
Added nested-hook and nested-component lint errors which detect nested hook or component syntax within component or hook syntax. This is on by default.

Notable bug fixes:

For default imports, the autoimport ranking will now consider the names of the importing side. (e.g. Previously we completely ignored the name of foo in import foo from './bar', but now we will count foo. If the pattern of import foo from './bar' happens a lot, then the autoimport algorithm will be more likely to suggest import foo from './bar' rather than import bar from './bar').
Flow will infer a correct type when viewing the type of an object literal as a dictionary type. For example, the error in this try-Flow will be raised.
Previously, we undercounted some imports during indexing, which causes autoimport ranking to behave incorrectly. The issue is now fixed.
Flow will no longer emit react-rule-hook-conditional error for hooks called in a conditionally defined nested component.

0.272.2

Notable bug fixes:

Prevent non-termination when computing code actions.

0.272.1

Notable bug fixes:

Fixed https://flow.org/try. It was broken from v0.270.0 to v0.272.0.
Documentation in hover now preserves indentations. Therefore, code blocks in jsdoc will no longer lose indentation.

0.272.0

Likely to cause new Flow errors:

When component syntax is enabled, hook calls happen inside an upper case function that doesn't have a props param that's a subtype of interface {} will get react-rule-hook-definitely-not-in-component-or-hook error.
Calling a function that's a union of hook and non-hook will now get react-rule-hook-mixed-with-non-hook error instead of react-rule-hook error. Calling functions named like hook but not in hook syntax will now get react-rule-hook-non-hook-syntax instead of react-rule-hook error.

Notable bug fixes:

Go-to-definition on default import of module.exports will correctly jump to the exporting file

Parser:

Fix crash on ''#!/usr/bin/env node\n'' when generating token list

Library Definitions:

React.forwardRef is marked as deprecated. We might remove it from our builtin libdef in the future.

0.271.0

Notable bug fixes:

Multiple levels of export * will now be correctly indexed so that they won't be missing in autoimport results.

Library Definitions

Add Float16Array type

0.270.0

Likely to cause new Flow errors:

When component syntax support is enabled, upper case functions with component-like name but doesn't return React.Node will no longer be treated like components. Thus, all the hooks call in such functions will have react-rule-hook-definitely-not-in-component-or-hook errors.
Usage of Object.assign will now trigger unsafe-object-assign lint error that's on by default. The type checking behavior for Object.assign will otherwise stay the same for now.
When Flow decides that the hook call definitely doesn't happen within component or hook body, it will emit errors with code react-rule-hook-definitely-not-in-component-or-hook.

Parser:

Parse the TS nonnull assertion operator.

0.269.1

It should have the same behavior as 0.269.0.

0.269.0

Likely to cause new Flow errors:

Using $Omit will trigger an internal-type error.
There might be some error code differences around destructuring
Object.assign now has stricter behavior over maybe type and optional type inputs. example

New Features:

You can now use this is T as type-guard annotation on a method m, to refine x when calling if (x.m()) {}. This-type guards can only be used on declare classes and interfaces. (E.g. try-Flow)
Added support for const type parameters that allow you to specify that an argument to a function is always treated as a const-expression, by annotating the type parameter with the const modifier. This eliminates the need to use the as const syntax when calling functions with this annotation. See more in our docs.

Notable bug fixes:

Auto-import for modules from module.system.node.root_relative_dirname will have correct import path instead of relative import path.
Auto-import for modules from @flowtyped will have correct import path instead of relative import path.
Generic render types are now allowed everywhere.
Fixed potential issue that makes hover on Omit<...> to fail.

Parser:

Parse class static blocks. The feature is not yet supported for type-checking.

0.268.0

Breaking:

The Linux x86 build is now built from ubuntu-22.04. It might make Flow not runnable on older linux distributions.

Likely to cause new Flow errors:

Code like the (example) will have [react-rule-hook-conditional] instead of [react-rule-hook-naming-convention] errors.
$Diff support is removed. If you have trouble migrating, you can try to polyfill it by this. However, $Diff has surprising behavior with regard to optional props in the second type parameter, which cannot be easily polyfilled (example).

New Features:

We now allow you to configure certain error code to be unsuppressable. For example, to make react-rule-hook-naming-convention and react-rule-hook-conditional errors unsuppressable, you can add the following to the [options] section in flowconfig:

unsuppressable_error_codes=react-rule-hook-naming-convention
unsuppressable_error_codes=react-rule-hook-conditional

0.267.0

Likely to cause new Flow errors:

We have updated the way type parameters are instantiated in generic calls, specifically when using upper bounds:
- We will no longer infer synthetic intersection types.
- If multiple upper bounds are available, we pick the smallest type based on subtyping (example).
Support for $Rest is removed. Omit should be used instead. If you still have many instances of $Rest, you can replace them with $Diff as a temporary measure, but note that we intend to eventually remove $Diff as well.
React-rule hook errors related to conditional hook calls will now have react-rule-hook-conditional error code.
React-rule hook errors related to naming convention issues will now have react-rule-hook-naming-convention error code.

Notable bug fixes:

We are rolling out the initial phase of a fix to a fundamental soundness issue related to primitive literal type inference. This unsoundness has allowed invalid code like: const x = 'a'; 'b' as typeof x as 'a'; (try-Flow) to type check without errors. With this fix, Flow will infer singleton literal types for primitive literals in contexts where such precision is required. Examples of this are: const-declarations (e.g. in const x = 42 will infer the type 42 for x), annotation positions (e.g. typeof x is equivalent to the type 42), conditionals (e.g. in if (x.tag === 42) {} Flow will infer the type 42 for the value 42, instead of number). In this part of the rollout, whenever this precision is not required Flow will infer the unsound type it used to infer before (a hybrid between the singleton and general type). Eliminating this unsound type completely will be done soon.
flow-remove-types now handles the removal of empty imports after removing type/typeof imports (thanks @jbroma)

0.266.1

Fix a bug that causes fixed libdef-override error to stick around after an incremental recheck.

0.266.0

Likely to cause new Flow errors:

Support for $PropertyType and $ElementType has been removed. Now referencing these types will just resolve to a global type if you have your own definition for them, and result in a cannot-resolve-name error otherwise. These types have been replaced by index access types for a long time. You can migrate to index access types by enabling use-indexed-access-type from https://www.npmjs.com/package/eslint-plugin-fb-flow and running the quickfixes. If you are unable to migrate, you can add the following to your global libdefs:

type $PropertyType<T, K> = T[K];
type $ElementType<T, K> = T[K];

Now given the subtyping check component()<: component(ref?: ref_prop), Flow will ensure that ref_prop is a subtype of void instead of a subtype of React.RefSetter<void>.
React$ComponentType, which was previously given [internal-type] error on every usage, is now removed.
React.ComponentType<Props> is now only an alias of component(...Props), instead of some special cased types. This comes with stricter checks and conversions, such as making Props readonly, erroring on the presence the ref prop instead of silently ignoring them, and ensures that Props is a subtype of {...}. In addition, the React$AbstractComponent type is removed.

Notable bug fixes:

fixed a subtle unsoundness in the inference of computed-property dictionary object creation (e.g. try-Flow)

Library Definitions:

React.lazy and React.memo is now generic over the presence or absence of ref prop.

0.265.3

Make @flowtyped resolution support work on Windows.

0.265.2

Notable bug fixes:

We fixed a bug that caused suppressions to not apply for libdef-override errors.
We fixed a bug that caused libdef-override errors not being raised even if it's turned on.

0.265.1

Notable bug fixes:

Make libdef-override=off in [lints] section actually turn off all libdef-override errors.

0.265.0

Likely to cause new Flow errors:

Overriding already defined names and modules in library definitions will now error with code [libdef-override]. The error cannot be suppressed without specific error code like $FlowFixMe[libdef-override]. It can be turned off by turning off the libdef-override lint.

New Features:

The Number static methods Number.isFinite, Number.isInteger, Number.isNaN, and Number.isSafeInteger now apply a refinement that their input is a number. Note that the top level isNaN and isFinite functions (not off of Number) do not apply the same refinement as they first coerce their input to number.

Notable bug fixes:

Fix a potential crash in libdef files with illegal import export. Thanks @techieshark for the repro.

0.264.0

Misc:

Version skipped.

0.263.0

Likely to cause new Flow errors:

Signature verification errors will now show up for libdef files

New Features:

Declaration merging for declare namespace is now supported in toplevel library definitions.

Notable bug fixes:

In component type annotation, the ref prop can now have any type.

Library Definitions:

Since the last version, most of the bundled libdefs will no longer be maintained and shipped with Flow. Going forward, they should be downloaded from flow-typed. Starting from this version, we will also no longer ship a set of precise typing definition for jsx intrinsics. To maintain the same behavior as before, you should have a flow-typed.config.json in the root of your project with the following content:

{
  "env": ["node", "dom", "bom", "intl", "cssom", "indexeddb", "serviceworkers", "webassembly", "jsx"]
}

0.262.0

Likely to cause new Flow errors:

Referencing React$Component directly will now be an error.
Use of React$ComponentType, React$Context and React$RefSetter will now trigger internal-type errors.

New Features:

Added types for JSX intrinsics, which will cause new errors and show autocomplete for DOM intrinsics like div

Notable bug fixes:

Replace incorrect definition for ClientRect and ClientRectList with aliases to DOMRect and DOMRectList This fixes the type definitions for the DOM APIs in Flow, but replacing an nonexistent globals ClientRect and ClientRectList types with just an alias to DOMRect and DOMRectList.
fixed a bug that caused spurious errors on rest array assignments (e.g. try-Flow).
When a library definition file has changed, Flow will no longer just stop the server. Instead, Flow will properly recheck everything, even under lazy mode.
We now allow something of type React.ComponentType<{}> to be a subtype of component()

IDE:

Component syntax components will now show up in document symbols

Library Definitions:

Most of the bundled libdefs will no longer be maintained and shipped with Flow. Going forward, they should be downloaded from flow-typed. To maintain the same behavior as before, you should have a flow-typed.config.json in the root of your project with the following content:

{
  "env": ["node", "dom", "bom", "intl", "cssom", "indexeddb", "serviceworkers", "webassembly"]
}

0.261.2

Notable bug fixes:

Fixed a bug that causes incorrect updates to our index that tracks usage of exports, which leads to incorrect autoimport-ranked-by-usage results.

0.261.1

Notable bug fixes:

Fixed a crash after initialization with saved state if the typing for react module is provided as a userland module.

0.261.0

New Features:

Bigint values inside of template literals now coerce to strings, e.g. `${1n}`. Addition with string (e.g. 1n + 'blah') is still banned.
Add ability to resolve node package entrypoints/exports
Flow now allows inference of type guards for simple expression arrow functions with one parameter. For example, in ["a", null].filter(s => s != null) it will automatically infer that the arrow function encodes a x is string type guard and will apply it on the input array resulting in aArray<string>. (try-Flow)
module.system.node.root_relative_dirname will allow conditional mapping like module.system.node.root_relative_dirname='<PROJECT_ROOT>/foo' -> 'bar'. Under such config, import 'a' will only be resolved to <PROJECT_ROOT>/bar/a if import 'a' is in a file in the <PROJECT_ROOT>/foo directory. This feature will be helpful if you want to combine two flow roots with different module.system.node.root_relative_dirname config.

Likely to cause new Flow errors:

When an opaque type is refined to be not null/undefined, we now refine the opaque type's upper bound rather than returning the opaque type unmodified.

Notable bug fixes:

Flow will error more consistently on incompatible uses of refined string, number and boolean literal types (e.g. try-Flow)
Fixed some cases of type filtering during type guard refinement (e.g. try-Flow)
Fixed a bug of missing errors when certain functions were checked against interfaces (e.g. try-Flow)
Fixed negation of refinement of type-guard functions with multiple parameters (e.g. try-Flow)
Relaxed type-guard consistency checks when the function returns with true or false. (e.g. try-Flow)

Parser:

We now provide parsing support for declare global {...} (using it will still result in an error). The AST will have the shape of

{
    "type":"DeclareNamespace",
    "loc": ...,
    "range": ...,
    "global":true,
    "id":{
        "type":"Identifier",
        "loc": ...,
        "range":...,
        "name":"global",
        "typeAnnotation":null,
        "optional":false
    },
    "body":{
        "type":"BlockStatement",
        ...
    },
}

Library Definitions:

The global React$PureComponent is removed. If you want to refer to it, it needs to be imported from react.

0.260.0

New Features:

You may now write object literals with Flow Enum values as computed keys. This will result in a dictionary type, keyed by the Flow Enum type. Note that all possible Flow Enum representation types are valid object keys, so we just support all of them.
You can use number typed keys (rather than just number literals) to create objects, these will be treated as dictionaries {[number]: T}

Notable bug fixes:

Fixed soundness hole in subtyping of type guard functions (e.g. try-Flow)
Fixed a bug that caused spurious errors when type guards were used in private methods (e.g. try-Flow)
Flow will no longer invalidate refinements made before the loop for const-like variables within a loop (example)

IDE:

For identifiers of array patterns bind to a require call, go-to-definition will jump to itself.

Library Definitions:

The internal type React$FragmentType is removed from Flow's builtin global type definitions.

0.259.1

Notable bug fixes:

Fixed a bug that causes experimental auto-import-on-paste to miss imports for component names used in jsx.

0.259.0

Likely to cause new Flow errors:

When a function without statics is passed to a place that expects inexact objects, the error will just say "function without statics is incompatible with object", instead of listing all the missing props in statics in errors. Error code might change.
When a non-callable object is passed to a place expecting functions, we will have a clearer message saying "non-callable object is incompatible with function" instead of the current error talking about the callable property. Some error code might change, which requires new suppressions.

Notable bug fixes:

For errors involving unions and intersections, we will now show a list of possible causes in a sorted order: the ones that are mostly likely to be the cause will be shown first.

IDE:

We slightly changed the go-to-definition behavior for export from statements:
- For bar in export {foo as bar} from '...', we will always jump to itself
- For foo in export {foo} from '...' or export {foo as bar} from '...', we will always jump to the name at the export, if this statement is well typed.
- For foo in export {foo} from '...' or export {foo as bar} from '...', we will jump to itself if the statement is not well-typed.
Go-to-definition on intrinsic jsx elements will jump to nowhere instead of jump to itself.
Go-to-definition for require('...') expression will now jump to the default export of an ESM module if available, or the first export of an ESM module.

Library Definitions:

Type for React.Context has been updated to support React 19's context as provider model. Given this change, you might need additional annotations on exports. e.g.

0.258.1

IDE:

Fixed go-to-definition for Meta-specific module references.

0.258.0

Likely to cause new Flow errors:

method-unbinding errors will not affect choice over intersection members. (e.g. try-Flow)
We now disallow coercions between functions and class statics.
The ability to configure react.ref_as_prop=disabled is removed.
Use of React$Node and React$MixedElement outside of libdefs will now trigger internal-type errors.

Library Definitions:

Improved definition of Array.concat().

0.257.1

Misc:

Flow language server will stop advertising that we can handle all code actions kinds prefixed with "source". It can help to prevent VSCode sending flow irrelevant code actions to handle during save.

0.257.0

Likely to cause new Flow errors:

We have improved inference for exported primitive literal values.
- Before this change the inferred type for export const obj = {f: "foo"}; was {f: str<"foo">}, where str<"foo"> was a type that opportunistically and unsoundly behaved either as string or as the singleton type "foo".
- Flow will now infer the type {f: string} for obj.
- For const like exports, e.g. export const x = "foo"; Flow will export the type "foo".
- To fix errors that arise due to this change, you can provide annotations around exported literals or use the as const modifier.
We fixed a major unsoundness with regards to dictionary object creation. Previously, the computed property will just be ignored for string or any keys. Now, if the computed property is added to some existing objects (e.g. {foo: string, [stringTypedKey]: v}), then we will error on the property. Otherwise, the {[key]: value} will be {[typeof key}: typeof value}
We fixed a bug that causes opaque type's underlying representation to leak beyond the file it's defined. You might see more errors as a result.
Flow will now perform a complete function call check even when [react-rule-unsafe-mutation] errors are raised.
Previously we incorrectly distribute-over-union for maybe or optional input types of conditional type. (e.g. (?string) extends string ? true : false is evaluated to true | false instead of false). This is now fixed, and code that depends on the bug might have new errors.
If you have refiend a value based on typeof x === 'function', and then do typeof x === 'object', the result is now empty as that is impossible.
Singleton types are now also considered in conditional equality checks on member expressions. (e.g. try-Flow)
React.Config type, deprecated in v0.254, is now removed.

New Features:

'key' in x now refines the type of x to objects which have the property key.
- in checks for both own and non-own properties.
- If we find that a key does not exist in an inexact object or instance/interface, then the negation is not refined since the property may or may not exist.
- An optional property is also considered as if it may or may not exist.
- If a proto/super is a union, every member of the union must have it
- If the input to the refinement is an intersection, one member of that intersection must have it
- We don't refine arrays since it's not useful (check for .length instead), and also because Flow doesn't handle array holes.
Flow now supports no_unchecked_indexed_access=true in flowconfig, which is equivalent to noUncheckedIndexedAccess from TS. It will add | void to every indexed access with general string key on dictionary objects or number key on arrays.
Support for React 19's ref-as-prop model is now available via react.ref_as_prop=partial_support, and this is now the default. (To disable it, use react.ref_as_prop=disabled.) Under this mode, ref prop in jsx will be treated as a regular prop for function components, but utility types like React.ElementConfig<...> still won't include the regular ref prop yet.
Flow now allows using union of strings as the type of computed property keys.
You can now destructure object properties using number literal keys (which are int-like).

Notable bug fixes:

Refining type guards with opaque types will now refine the general type (example of code that used to fail before, but now passes: try-Flow)
Allow more flexibility in checks involving super type of opaque type (e.g. try-Flow)
Fixed a bug when using opaque types in type guards in files other than the one where the type guard function was defined.
We now allow a union of generic or opaque string typed values to be used as a key of computed property, if it's bounded by a string type.

Library Definitions:

Add libdef types for getNotifications and showNotification API for ServiceWorkerRegistration

0.256.0

Notable bug fixes:

Fixed a limitation when checking intersections of polymorphic types (e.g. tryFlow)
Fixed an internal error raised when using super in an object method. Such cases are now user-facing errors.

0.255.0

Likely to cause new Flow errors:

Referencing a react-component typed value in type position will now create a React$RendersExactly<typeof Component> type instead of React$Element<typeof Component> type. All existing usages intended to create an exact react element will now error as a result. Using it as type passed to render type, React.PropsOf, React.PropOf and React.RefOf is unaffected and will not be affected in the future. If you want exact react elements, the only way that doesn't trigger Flow error is by ExactReactElement_DEPRECATED<typeof Component>.

Library Definitions:

React.Config type is marked as deprecated and will be removed soon. You can create your own equivalent type with

type ReactConfigShim<Props, DefaultProps> = $ReadOnly<{
  ...Omit<Props, $Keys<DefaultProps>>, ...Partial<DefaultProps>
}>;

0.254.2

Misc:

Add option to disable experimental configuration

0.254.1

Notable bug fixes:

Fixed a potential crash when experimental multiplatform support is enabled.

0.254.0

Likely to cause new Flow errors:

Previously we incorrectly applied truthy refinement on optional chaining where it shouldn't be applied. The issue is now fixed, and you might see change in behavior similar to these examples

Library Definitions:

Improve Array toSpliced method type (thanks @aph!)

0.253.0

Likely to cause new Flow errors:

React$ElementRef is now implemented with a conditional type instead of a builtin Flow utility type. You might see new errors with generic React$ElementRef. Most of the times, you should replace React.ElementRef<T> with just T if there are no other uses of generic T.
We've increased safety in checks involving recursive types.

New Features:

You can now set multiple @flowtyped like directories via module.declaration_dirnames config. e.g.

module.declaration_dirnames=<PROJECT_ROOT>/decl1
module.declaration_dirnames=<PROJECT_ROOT>/decl2

Please note that when you set this, @flowtyped at the root will no longer be considered special, and you will need to explicitly add it like

module.declaration_dirnames=<PROJECT_ROOT>/@flowtyped

Notable bug fixes:

A generic type as input to conditional type will have type parameters pinned with their default or upper bound, instead of being empty.
We fixed a cache collision issue that might cause instantiation of a value-imported generic component type to interfere with a typeof-imported generic component type.

IDE:

Flow now respects includeDeclaration from LSP request for find-references command. In practice, this means that "find all references" in vscode will continue to return both declaration and all uses, while "go to references" will exclude the declaration.

0.252.0

Likely to cause new Flow errors:

Flow will no longer apply type transformation to function components' props when you extract props through the utility types. For example, function components' props won't automatically get $ReadOnly applied, which might cause new errors.

New Features:

We have removed the restriction of not allowing generics with ref prop in component syntax

Notable bug fixes:

Files in [declarations] section will have all lint errors silenced.
Fixed an issue that caused errors in hover type on unions of evaluated types (example)
Flow allows unions of strings to be used as the key in creation of objects with computed properties.

0.251.1

Misc:

Performance fix

0.251.0

Likely to cause new Flow errors:

Remove the deprecated React.AbstractComponent type. We recommend replacing them with component types. Since v0.249.0, we have provided a codemod via the flow-upgrade package to aid larger codebases with this conversion: yarn run flow-codemod eliminateAbstractComponent path/to/src.
In addition to the removal of React.AbstractComponent type, we also removed support for the internal React$AbstractComponent when more than 1 type argument has been passed. Similarly, you should migrate to use component types if you want to specify ref props and renders.

New Features:

We now support a new way to easily specify library definitions for npm modules. For all files in @flowtyped directory at the root, they can be resolved by the relative path without leading ./ relative to @flowtyped. e.g. <PROJECT_ROOT>/@flowtyped/react.js.flow can be used to type the react package, and <PROJECT_ROOT>/@flowtyped/react/jsx-runtime.js.flow can be used to type react/jsx-runtime module. We now recommend using this approach instead of using declare module in global libdef.

Notable bug fixes:

any will now correctly propagate to inferred render type. example.
Fixed a bug with regard to jsx type inference with generic callable objects. example
Fixed a soundness hole that allowed frozen objects to be incompatible with non-readonly objects. This example will now produce an error.

Library Definitions:

We have updated some core React type definitions to use the new component type instead of React.AbstractComponent. Most of the code won't be affected, but you might have to sometimes make the Props readonly.

0.250.0

Likely to cause new Flow errors:

Fix a bug where scope analysis of exported component type is done incorrectly, which causes generics mentioned within component type to be incorrectly typed as any.

Notable bug fixes:

We will now remove previous props in generic component types in type argument inference. example
When all option is specified in the config of flow-remove-types, we now respect it and will correctly handle Flow-typed files without @flow pragma.

0.249.0

Likely to cause new Flow errors:

Uses of React$AbstractComponent outside library definitions will now trigger internal-type lint error, which is on by default. React.AbstractComponent is also marked as deprecated. We recommend replacing them with component types. We have provided a codemod via the flow-upgrade package to aid larger codebases with this conversion: yarn run flow-codemod eliminateAbstractComponent path/to/src.

Notable bug fixes:

Fixed issue with StringPrefix and StringSuffix when used as a component syntax prop.
Fixed an issue that causes type argument inference on component type to be incorrectly under-constrained. (example)
Fixed as cast support in flow-remove-types.

Library Definitions:

React string refs are no longer supported in React.cloneElement. e.g. React.cloneElement(e, {ref: 'foo'}) will be an error.
The deprecated React$Ref type and various aliases of it are removed.

0.248.1

IDE:

React.Component annotated exported will now also show up in auto-imports in a type position.

0.248.0

Breaking changes:

Support for long deprecated predicate function (%checks) is removed. It will still parse, but all of them will error with unsupported-syntax, and %checks will be completely ignored for type checking purpose as if it doesn't exist.
$TupleMap support is now removed. $TupleMap will now resolve to the global definition is there is one, or it will fail and the type becomes any.

Likely to cause new Flow errors:

Support for the unsound $TEMPORARY$* types is dropped. If your codebase happens to have any of these types, you can replace them with there sound and well-documented equivalents:
- Replace $TEMPORARY$object<{props}> with $ReadOnly<{props}> or {props}
- Replace $TEMPORARY$array<T> with $ReadOnlyArray<T> or Array<T>
- Replace $TEMPORARY$number<42> with number or '42'
- Replace $TEMPORARY$string<"foo"> with string or "foo"
- We have provided a flow-runner codemod via the flow-upgrade package to aid larger codebases with this conversion: yarn run flow-codemod replaceTemporaryTypes path/to/src.
The inferred type for Object.freeze({ A: 'a', B: 'b' }) is now {+A:'a',+B:'b'} both locally within a file and when the object is being exported. This replaces an earlier unsound behavior where the type of A would opportunistically behaved either as string or 'a' depending on the context where it appeared.
React string refs are now banned.
contextTypes and childContextTypes in react class components are now empty typed, so declaring legacy context in React will be an error.
Component syntax component and component types' ref prop now must have React.RefSetter<...> type.

Parser:

Component type in parentheses can now be correctly parsed. e.g. type Foo = (component(x: number) renders Bar);
Trailing comma is now allowed after rest parameter in component syntax components and component types.
The v regex flag is now supported.

IDE:

We now provide a code action to stub out a react component, at the location of an unbound JSX identifier.
Component declaration/type without renders clause will no longer show renders React.Node on hover.
Hovering on components will now consistently show its props and renders information. Previously, the information is omitted for component declarations.
If we see : renders <annot> at the position that expects a render declaration, the quickfix will suggest removing : instead of replacing : with renders.

Library Definitions:

Added type for util.stripVTControlCharacters for NodeJS.

0.247.1

Misc:

Performance optimization for unions

0.247.0

Breaking changes:

Support for the deprecated $Call type is removed. $Call will now resolve to whatever $Call points to in the global libdef. If you need more time to migrate, you can create a shim like $Call<F, T> = $TupleMap<[T], F>[0], but please note that we intend to remove support for $TupleMap eventually.
deprecated-type-dollar-call lint is removed, since the support for $Call is removed.
react.disable_function_components_default_props config option is removed. It is on by default since v0.238.

Likely to cause new Flow errors:

$TupleMap is deprecated and will be soon removed, now that mapped type works on array inputs. Existing $TupleMap will still work in this release, but every use will trigger a deprecated-type lint that is on by default.
Flow now performs literal subtyping checks for strict equality conditions in non-refinement contexts. example
Fixed destructuring with invalid literal defaults. The following now errors properly: example
Using string ref on components that are definitely not a class component is now an error. example
React utility types will no longer incorrectly accept hook types when they expect component types. e.g. type A = React$ElementConfig<hook (props: {foo: string}) => void>; will now error.

New Features:

Refinements against negated bigint literals should now work.

Notable bug fixes:

Mapped type on generic arrays is now supported. Previously it will fail with array is not an object error.
flow-remove-types now correctly handles as cast with generics.

IDE:

On hover, values that have React.AbstractComponent type will be shown in the component type syntax
Flow now offers a code action to insert inferred render type when you hover on the name of the component.
Flow now provides keyword completion for component type, hook types and render types.

Library Definitions:

Overly restrictive typing of queueMicrotask is now fixed.

0.246.0

Likely to cause new Flow errors:

Support for $ObjMap has been completely removed. This change means that:
- $ObjMap will resolve to a user-defined $ObjMap global if it's available in your libdef. You can use type $ObjMap<O, F> = {[K in keyof O]: $Call<F, O[K]>} to get most of the same behavior, except that in the mapped type version O[K] will include void for optional props. Please note that this should be considered as a permanent shim, because we intend to eventually remove $Call.
- flow codemod key-mirror command was removed, since this codemod tries to convert $ObjMap to $KeyMirror
- deprecated-type-objmap lint was removed.
Component syntax components without ref prop will have void as the instance, so React.ElementRef<component> of such component will return void.

New Features:

Mapped types now support array and tuple types in the form of {[K in keyof <array or tuple type>: <mapped type>}.
- With this support, we intend to deprecate $TupleMap in the next release and remove the support for $TupleMap eventually
You can now refine against variables and member expressions with a literal type, not just literal themselves.

Notable bug fixes:

Flow now only reports one error when two disjoint large enum-like unions are compared (example).

IDE:

Code actions on component syntax components will produce smaller targeted edits, rather than reprint the entire component.
Deprecated utility type like $Call are no longer suggested in autocomplete.
Flow will now provide a quick fix when the spread prop of a component with component syntax redefines some already defined props.

Library Definitions:

React.Ref is marked as deprecated. Use React.RefSetter instead for ref props, and React.RefObject for returns of useRef.
React.RefSetter now includes null and void.
Promise.all and Promise.allSettled have been updated to use mapped type instead of $TupleMap. The requirement on the type arguments are slightly changed.

0.245.2

Misc:

The language for invalidated refinement is slightly tweaked. Instead of saying refactoring to a constant, we now say refactoring to a const variable.

0.245.1

Website:

Refined expressions are now highlighted in try-flow playground. Hovering on refined expressions will show where it's refined, and hovering on some invalidated property refinements will show the location and reason of the invalidation.

IDE:

Flow now responds to textDocument/prepareName LSP request. This ensures that Flow's rename support can be surfaced when multiple VSCode extension adds rename support for JS files.

0.245.0

Likely to cause new Flow errors:

React.Element type, deprecated in 0.243.0, is now removed.
Fixed a bug where refinements are incorrectly not invalidated. example
Under custom jsx pragma, children is no longer incorrectly added to props. Some spurious errors will be gone. example

New Features:

Added LSP signature help support for JSX attributes. The feature is triggered with the opening { of a JSX attribute value.
Flow now allows to configure the typing of jsx creation under the new option react.custom_jsx_typing=true. Under this flag, Flow will type check jsx by through your self-defined global type React$CustomJSXFactory type, which should be a function type that accepts arguments according to jsx spec. e.g. You can write a loose one like type React$CustomJSXFactory = (component: any, props: any, ...children: any) => React.MixedElement

Notable bug fixes:

Fixed jsdoc attachment to signature help results for overloaded functions. (example)
Signature help on overloaded functions will show information in the correct order of signatures
Labels of autocomplete on members will indicate if the property is optional with a "?"
Fixed a bug where a type-only namespace within a namespace is accidentally dropped. This bug used to make globalThis.React.Node being incorrectly unavailable.
Fixed poor interaction of "Add missing attributes" quickfix when Linked Editing Range is enabled.
Clicking on related symbol location on hover will jump to the right location in VSCode. Previously it incorrectly jumped to the position 1 character left of the right location.

IDE:

Elements of component syntax components (e.g. typeof <A /> where A is in component syntax) will no longer be shown in hover with shorthand syntax like A. Instead, it will be shown as React$Element<typeof A>

0.244.0

Likely to cause new Flow errors:

The inexact_tuple_types_syntax option (which was on by default) is now deleted. Make sure you are using hermes-parser and related packages at version >= 0.23.0
Render types will no longer allow specific renders wrapping more general renders like renders React.Element<React.AbstractComponent<{}, mixed, renders* A>>. Invalid render types will behave like any.
Some previously allowed generic render types are no longer allowed (example). This ensures that generic render types are only used to model transparent components like React.Fragment.
renders* T where T is generic is now banned.

Notable bug fixes:

Render type with union type arguments will be correctly wrapped with parentheses.

0.243.0

Likely to cause new Flow errors:

All deprecated-type and untyped-type-import lint are on and at error level by default.
Use of some internal types of Flow that have other identical public-facing variants (e.g. React$ElementConfig vs React.ElementConfig) are now a lint error that's enabled by default. To disable it, set internal-type=off in the lints section of the flowconfig. For now, these types are still allowed in libdef files, but we plan to also disallow it in the future. To codemod existing code, get the latest flow-upgrade package and run yarn run flow-codemod replaceReactDollarUtilityTypes
Direct use of React$Element type is banned via the internal-type lint. The React.Element alias still exists for now, but it is marked with @deprecated in the jsdoc. Please read the jsdoc for better alternatives. A global alias ExactReactElement_DEPRECATED is added to help you clearly mark the use of exact React element types, and you should use it to replace React.Element that you cannot replace without compromising runtime type safety.
Flow now consistently errors on bad comparison with enums example

New Features:

Allow generic bound by inexact tuple as function rest args. This example now works.
Support for declare component statement and component type is enabled by default.
Flow now provide the "insert inferred type as type cast" code action. You only need to select the expression in the editor to trigger the code action.

Notable bug fixes:

Fixed a bug that causes an internal error. example

Misc:

experimental.namespaces config option is removed. Namespaces support is enabled by default in the previous release.

Library Definitions:

Add .replaceChildren method type definition for dom nodes.
Object.getPrototypeof is still special cased, but the special-casing is only available for a syntactic Object.getPrototypeof(...) call. If you try to use Object.getPrototypeof in some other ways (e.g. const f = Object.getPrototypeof; f(...)), you will get a a less precise and accurate typing for it. The Object$GetPrototypeOf type, which was used to back the special case behavior, is removed.

0.242.1

New Features:

For Pick utility type with string literals in the second type argument, go-to-definition on the string literal will jump to corresponding prop's location

Notable bug fixes:

Re-exported types will show in autoimports from files without any import statement.

0.242.0

Likely to cause new Flow errors:

Support for special function type $Flow$DebugPrint was removed. To see the internal representation of type, use the flow type-at-pos command with flag --debug-print-internal-repr. Note that this is for debugging purpose only, and you should never depend on the output of --debug-print-internal-repr
More invalid compare errors will be consistently caught when using == and !=.
invalid-render errors can no longer be suppressed without specific error code.
Flow can now detect more bad cyclic types, including across module boundaries. example
You may see some errors moved around when $Exact<...> is involved. Some invalid $Exact<...> type will now have not-an-object error code.
Fixed a bug that makes some exported types accidentally any. Previously hidden errors might be exposed.

New Features:

declare namespace support is enabled by default.
Added "Add Missing Attributes" quickfix for JSX. This is available when the cursor is over the name of a JSX element that is missing a required attribute.
Added a StringSuffix type, which goes along with the recently added StringPrefix type. StringSuffix can be used to type strings with a specified suffix. E.g. StringSuffix<'!'> allows for 'yay!' and 'woo!'. The type argument for the suffix must be a string literal. The second, optional, type argument can be used for the type of the remainder of the string after the suffix is removed.

Notable bug fixes:

Fix a bug that caused Flow to infer incorrect render types for elements of polymorphic components under niche circumstances

IDE:

Flow now supports symlinked node_modules in autoimports better. If allowed by the node resolution algorithm, autoimport will insert import specifier like my-package or my-package/foo instead of a relative path import.

Library Definitions:

CredMgmtCredentialRequestOptions properties are now optional
Reflect.setPrototypeof are no longer special-cased, and the backing special type Object$SetPrototypeOf is removed. The new typing is intentionally loose. Similar to Object.setPrototypeof, this is inherently unsafe since Flow won't track prototype mutations.
Object.assign is still special cased, but the special-casing is only available for a syntactic Object.assign(...) call. If you try to use Object.assign in some other ways (e.g. const f = Object.assign; f(...)), you will get a a less precise and accurate typing for it. If you don't like the default fallback type, you can override the Object$Assign type. While the special-casing behavior will stay for the syntactic call, you should migrate your code to use the spread syntax instead.

0.241.0

No changes from 0.240.0

0.240.0

Likely to cause new Flow errors:

We fixed a bug that causes Flow to incorrectly believe that the LHS of logical operators are not nullable. example
Flow can now detect more kinds of bad cyclic types. (example)
Flow will now consistently catch some invalid comparison of the kind obj.type === 'foo'. example
Fixed a bug in the type guard refinement that involves sentinel properties (e.g. try-Flow)
Function.prototype.apply and Function.prototype.call are now typed using userland types instead of hardcoded builtin support. You might see slight inference differences, error message and code changing as a result.
Flow now enforces that only function types are assigned to the .constructor property of object types. (D58538540 pvekris)
Fix subtyping checks of unions of bigint types against other unions of bigints.
Make array holes be typed as void, not empty.

New Features:

You can now pass in an (optional) second type argument to the StringPrefix utility type. This is the type of the remainder of the string after the prefix is removed. For example, "$1", "$2" are subtypes of StringPrefix<'$', '1' | '2'>, but "$999" is not. StringPrefix<X> is equivalent to StringPrefix<X, string>
You can use the $Values utility type to get the type of the elements of an array type or tuple type.
You can now do sentinel refinements on tuples, like you can already do for objects. This is especially useful for function arguments (you can type the ...args as a union of tuples). [example]
Sentinel refinements now work when using number keys on objects [example]
Flow now keep original type after invalid object prop-exists refinement on write-only property.

Notable bug fixes:

Some operations can now be performed on an opaque type with appropriate bounds. example
Fixed a bug that occurs when you have a tuple type spread after two or more non-spread arguments. Those will now be resolved in the correct order.

Library Definitions:

String.codePointAt now returns number | void

0.239.1

Notable bug fixes:

Fixed a bug where IDE services doesn't respect files.implicitly_include_root=false config.

0.239.0

Likely to cause new Flow errors:

We now detect errors when props of React components using component syntax, or hook argument/return values of React hooks using hook syntax, contain arrays, sets, or maps that are mutated using their methods (Array.push, Set.add, Map.set, etc). These values are expected to be read-only, and we previously errored on directly setting their props; this release extends this enforcement to method calls as well.
We are adding more strict checking for type guard functions to account for the refinement happening in the else branch of conditionals that use them (see docs for more information). You might see new errors appear in the declaration of type guards. One way to address these is by turning the type guard definition to one-sided, by adding implies before the type guard (example try-Flow)

New Features:

The StringPrefix type represents strings which being with the specified prefix. E.g. StringPrefix<'data-'> allows for 'data-floo' and 'data-bar'. The type argument for this type must be a string literal. [example]
Flow now supports globalThis.

Notable bug fixes:

Fixed an issue since 0.232.0 that will cause failure to connect to Flow server if libdef has parse errors.
Made the "extract to function" refactoring more robust to errors during code synthesis.
Fixed a bug that can cause hover to hang forever for recursive namespaces. example
Go-to-definition on new C() will jump to definition of C instead of the constructor definition. Hovering on C will still show the jsdoc on the constructor.
Strip as const casts and as casts in flow-remove-types

IDE:

Hover will show a list of all the symbols found in the inferred type and the locations where they are defined. VSCode LSP and CLI are supported. The LSP version includes a link to the definition. The CLI version only shows the name of the file (no path)

Library Definitions:

Calling shift() and pop() on an Array<T> will now return T | undefined.
Add cause property on Error instance; Support error cause in error constructor options
Add type definition for FinalizationRegistry
Add type definition for CSSSupportsRule
Add closeAllConnections, closeIdleConnections to https$Server

0.238.3

Misc:

In v0.238.1 and v0.238.2, we have bumped the required GLIBC version to v2.35 on Linux. We have now reduced the requirement to v2.31 for x86_64 build of Linux.

0.238.2

Notable bug fixes:

Fixed a bug where a recheck triggered by flow force-recheck doesn't respect files.implicitly_include_root=false config.
Fixed a bug that causes missing results in find-ref and rename.

0.238.1

This release is used to test the release automation powered by GitHub Actions. No changes are expected.

0.238.0

Likely to cause new Flow errors:

In v0.237.2, we shipped the flag react.disable_function_components_default_props that will make Flow ignore defaultProps on function components for React component typing purposes. This flag is now on by default. If this is too disruptive for your codebase, you can turn this off for now. Note that we do not intend to keep this flag forever, so you should try to turn this flag on as soon as possible.
We made React.createElement have an opaque type, which means calling this function directly will be an error. This will be helpful to prepare for React 19 changes. Note that JSX typing is unaffected. If you are not ready for this change, you can override your library definition like this to approximate the previous behavior as much as possible.
When there is an invalid-component-prop error, Flow will no longer make the entire props to be any type. Instead, Flow will create a Props type as if these invalid props do not exist. example
We are making the typing of the exported object of a module more strict. If you see errors when casting the exported type of a module to a writable object, try either casting it to a readonly version instead, or casting the exported object to the writable object type.
We now infer the type for Object.freeze({ A: "a", B: "b" }) as {+A: "a", +B: "b"}, ie we use readonly properties and singleton types for literal initializers. Casts like this example will now be an error.
We now error more consistently when a property is missing from an intersection of objects (e.g. try-Flow)
untyped-import errors are now on the import source rather than imported names.

New Features:

Under files.implicitly_include_root=false (default is true, which is the current behavior), Flow will no longer include everything under the directory where the flowconfig is in by default.

Notable bug fixes:

Fixed a category of spurious errors when using Array.reduce and returning a union type (e.g. try-Flow)
Explicit type arguments in JSX are now considered for contextual typing. example
Fixed spread of empty array when calculating tuple elements, e.g. [...[]] as [] now works.
nested-component errors can now be suppressed.

IDE:

Flow now suggests properties of objects that are contextually typed with optional types. For example, in ({ foo: { | } } as ?{ foo: { x: number }}) it will populate x, when calling autocomplete at the point of the cursor |. (try-Flow)
We now provide autocomplete for types in the global React type-only namespace.
We now support go-to-definition for JSX member expression like <Foo.Bar />.
Re-exported values from export {Foo} from 'bar' statements will now appear in autoimport results.
autoimports_ranked_by_usage is now enabled by default.
Flow will now show the type of the constructor of a class (instantiated if it is generic) when hovering over the "new" keyword in new C(e)
Hover types will now show the specialized version of a polymorphic component used at a JSX expression
Hover types won't add unnecessary parentheses around union types.

Library Definitions:

Add React 19 useOptimistic and useActionState API definitions
Add libdef for TextDecoderStream

0.237.2

New Features:

We added a flag to help prepare your codebase for React 19's defaultProps change. defaultProps on function components will no longer work in React 19. Under react.disable_function_components_default_props=true, Flow will ignore defaultProps on function components for React typing purposes. Note that we do not intend to keep this flag forever, so you should try to turn this flag on as soon as possible.

0.237.1

Notable bug fixes:

Fixed an issue that might cause connecting to flow server to fail with out of retries.

0.237.0

Likely to cause new Flow errors:

Flow will no longer break up a union argument when calling an overloaded function. The code in this try-Flow will now be an error.

New Features:

Under jest_integration=true, Flow will now error on some jest mocking APIs (e.g. jest.mock('./my-module')) if the module doesn't exist.

Notable bug fixes:

Fixed spread of empty array when calculating tuple elements, e.g. [...[]] as [] now works.
Fixed inferred type of React.ElementConfig/Props when passed in a component syntax component (e.g. try-Flow)

0.236.0

Likely to cause new Flow errors:

Ensure React class component constructors call super with a props object. example
Modified typing of React.cloneElement. You might see errors being shifted around, and some advanced patterns no longer supported. The API is not encouraged by React, so migrating away from it is recommended.
For JSX elements under react.runtime=classic, we now type check the JSX unconditionally using the right React.createElement typing, and check whether the locally defined React has the right React.createElement definition. If you have some JSX that's already invalid, you might see different errors. example
React$CreateClass type, aliased to any since v0.176, is removed.
Fix typing of isValid Flow Enums method, reverting back to behavior from before v0.232.
Support for $CharSet, an undocumented feature, has been removed. RegExp$flags, which depends on $CharSet, is also removed. In most cases, you should replace these types with string.
Support for --traces and the corresponding flowconfig option has been removed.
React.createFactory libdef is removed. This API is already deprecated and will be removed in React 19.

Notable bug fixes:

Fixed interactions of various types with specialized versions of mixed (such as $NonMaybeType<mixed>). (try-Flow examples)

Misc:

Support removing export type * (ExportAllDeclaration) nodes with flow-remove-types (thanks @jbroma!)

Parser:

Updated bigint literal AST output for bigint property to match ESTree spec. Numeric separators are removed (_), and should contain only decimal digits.

IDE:

Under component_syntax=true, autocomplete will provide component and hook keyword in appropriate places.

Library Definitions:

$asyncIterator, which is never a real global, is removed from global libdef.
We removed $await in the libdef. If you are depending on it like typeof $await, you can replace it with <T>(Promise<T> | T) => T.

0.235.1

Misc:

By default, flow ast will now parse component syntax rather than reject it. You can pass --no-component-syntax to get the old behavior (D56501290 samzhou19815)

0.235.0

Likely to cause new Flow errors:

Flow's react-rule enforcement now detects reads from ref.current within nested functions of hooks and components that are called during rendering.
obj[key] where obj is not a dictionary object and key is a string used to not error and just silently return any. We now error on this pattern.

0.234.0

Likely to cause new Flow errors:

Flow might catch more inexact incompatible with exact errors. example
Fixed a bug that leads to Flow sometimes ignoring differences in call props of an object. New errors might be exposed. example
We rewrite the way we generate types for annotations. We will now detect and error on trivially recursive types like type T = T or type Foo = typeof foo; const foo: Foo = .... In addition to this, you might see some errors being moved around.
Fixed Flow Enums exhaustive checking logic when input is a generic.
Invalid indexed access types with string index, like {foo: string}[string], will now error instead of silently making it any.

New Features:

You can have a tuple spread of a generic in the parameter of a function and not have to supply the relevant type argument if that spread is the only spread in that tuple, and it is the last element of that tuple.
Negative numbers are now allowed in Flow Enum values.
Allow Flow Enums to be cast to their representation type when the cast expression is typed as a generic type.
Added a new global type EnumValue<>, which represents all Flow Enum values. You can constrain the enum values to those with a particular representation type by supplying the type argument, e.g. EnumValue<string>.
Added a new global type Enum<>, which represents all Flow Enums (this is a complement to the new EnumValue type). You can constrain the enums to ones which have a particular enum value, e.g. Enum<> is the same as Enum<EnumValue<>>, and the type of all enums with string representation types is Enum<EnumValue<string>>. These "abstract enums" have no know members, so you can't access members or exhaustively check them, but you can use methods such as .members(), .cast(), etc.
Allow === comparison of abstract Flow Enums as well as Enums themselves with the same ID.
You can now cast Flow Enum values to their representation type using .valueOf(), e.g. enum E {A, B}; const s: string = E.A.valueOf().

Notable bug fixes:

We now error more reliably in the matching property tests, for example when the object part is a complex expression. example
Fixed spurious errors in propagating type hints. example
Infer type in a conditional type is now allowed to be underconstrained. When the true branch is taken, this underconstrained infer type will be pinned to the upper bound. example
Add Enum and EnumValue to $NotNullOrVoid - previously there was no supertype for all enum values or enums.

IDE:

We now show jsdoc information on component props with component syntax.

0.233.0

Likely to cause new Flow errors:

Flow will now error on invalid type applications at the declaration site, if the type application does not use another generic type from an outer scope. example
We fixed a bug where we incorrectly unwrapped the opaque type to the underlying representation in refinements
Predicate functions (%checks) can no longer be recursive
Fixed subtyping of indexers in instances/interfaces

Notable bug fixes:

We fixed some spurious errors regarding opaque type refinements. example
We made a change to method-unbinding errors so that it no longer errors when in an any/mixed context. This allows us to better support jest
We fixed an issue where explicitly supplied JSX type arguments were ignored
We fixed bigint Flow Enums that do not explicitly speicy of bigint during parsing. We were previously not correctly inferring that this was a bigint enum.
flow.org/try now supports go-to-definition, autocomplete, and signature help

Parser:

Previously Flow outputs ReadOnlyTypeAnnotation in parser output for annotations like readonly string[]. Now we output it as TypeOperator with "operator": "readonly"

IDE:

Fix printer output of casting syntax when LHS of as or as const has low precedence

Library Definitions:

We migrated the built-in React library definitions to use component and hook syntax for various React-provided APIs. Look out for a blog post on our medium account for more details and see the React section in our docs

0.232.0

Likely to cause new Flow errors:

Support for $Compose and $ComposeReverse types are removed. We recommend to use overloads to approximate their behavior instead. e.g.

declare export function compose(): <T>(a: T) => T;
declare export function compose<F: (...$ReadOnlyArray<empty>) => mixed>(
  f: F,
): F;
declare export function compose<A, T: $ReadOnlyArray<any>, R>(
  f1: (a: A) => R,
  f2: (...T) => A,
): (...T) => R;
declare export function compose<A, B, T: $ReadOnlyArray<any>, R>(
  f1: (b: B) => R,
  f2: (a: A) => B,
  f3: (...T) => A,
): (...T) => R;
declare export function compose<A, B, C, T: $ReadOnlyArray<any>, R>(
  f1: (c: C) => R,
  f2: (b: B) => C,
  f3: (a: A) => B,
  f4: (...T) => A,
): (...T) => R;
declare export function compose<R>(
  f1: (b: any) => R,
  ...funcs: $ReadOnlyArray<(...$ReadOnlyArray<empty>) => mixed>
): (...$ReadOnlyArray<any>) => R;
declare export function compose<R>(
  ...funcs: $ReadOnlyArray<(...$ReadOnlyArray<empty>) => mixed>
): (...$ReadOnlyArray<any>) => R;

You might see more errors around type application. example
Fix subtyping of indexers in instances/interfaces. example

New Features:

Updated the isValid Flow Enums method to use a type guard, allowing it to refine its input to the enum type in a conditional context. E.g.

enum Status {Active, Off}
const s = "Active";
if (Status.isValid(s)) {
  s as Status; // Should work
}

export type Foo = ... and export interface Bar {...} statements are now allowed in declare module and declare namespace bodies.
We added a new codemod flow codemod remove-unnecessary-react-import which can help remove unnecessary react imports under react.runtime=automatic

Notable bug fixes:

Fixed issue when explicitly supplied JSX type arguments are ignored
Fixed code action output of casting syntax when LHS of as or as const has low precedence

Library Definitions:

Added definitions for the following APIs:
- window.showOpenFilePicker(): MDN, WICG
- window.showSaveFilePicker(): MDN, WICG
- window.showDirectoryPicker(): MDN, WICG
- DataTransferItem.getAsFileSystemHandle(): MDN, WICG
- StorageManager.getDirectory(): MDN, WHATWG

0.231.0

Likely to cause new Flow errors:

Fixed subtyping of inline interfaces, e.g. try-flow.

New Features:

No longer trigger method-unbinding errors with indexed access types.
Add relay_integration.esmodules option. When this option, along with relay_integration, is enabled, Flow will treat graphql fragments/queries/etc. as import default rather than require. Use this when you output ESM from Relay's codegen (e.g. eagerEsModules).

Notable bug fixes:

Don't error when returning an array, map, etc when an AsyncGenerator<T, void, void> type is expected (e.g. try-Flow)

IDE:

Fixed a bug where AUTO332 appeared in the result of string literal type completion.

Library Definitions:

Added definitions for FileSystem APIs (thanks @ZelJin).
Added constructor to the StorageManager class (thanks @ZelJin).
Removed checkPropTypes, DOM, createClass, ConcurrentMode from react module.
Types in react module can now also be used when React is default imported like import React from 'react', they can also be used without imports, e.g. type N = React.Node.

0.230.0

Likely to cause new Flow errors:

We now put value exports and type exports into two different namespaces. It means that we will now consistently error on places where you try to import, re-export, or use a imported type as a value. This change also makes certain patterns that happen to work in the past no longer work, e.g.

// a.js
export type Foo = string;
// b.js
const A = require('./a');
module.exports = {...A};
// c.js
// previously allowed, no longer allowed
import type {Foo} from './b';

We have allowed function parameters to refer to each other for a while now. However, we didn't implement the analysis correctly for exported functions, which causes inconsistent behaviors. This is now fixed e.g.

// a.js
// we incorrectly resolve x to refer to
// be external to the function params,
// which is a global in this case
export function foo(x: string, y: typeof x) {}
// b.js
import {foo} from './a';
foo("", 3); // no error before, errors now

Previously, we allowed assignment to any globals, regardless of whether it's a type or value. Therefore, both were allowed in Array = 3; $ReadOnlyArray = 2; Now the latter is banned.
Flow now consistently bans referring a type-only global as value
Some bad annotations that cause value-as-type or recursive-definition errors will no longer affect which branch to take in overload resolution.

New Features:

Flow now supports typeof with type arguments type T = typeof foo<string>. This syntax is supported in prettier since v3.1. If you previously enabled the flag typeof_with_type_arguments=true, you need to remove it.
Flow now supports explicit type arguments on JSX. e.g. <Foo<string, _, number >propA='d' />. Note that some type arguments can be inferred using the _ syntax.
Flow now supports the same NoInfer intrinsic that will be available in TypeScript 5.4.
Under experimental.ts_syntax=true, Flow will
- Automatically translate TypeScript's readonly on tuple and array types into Flow equivalent $ReadOnly<[...]> and $ReadOnlyArray<...> without error.
- Automatically translate TypeScript's keyof into Flow equivalent $Keys without error. Note that there are behavior differences between $Keys<> and keyof right now (flow, TS), so use it with caution.
- Automatically translate TypeScript's unknown never and undefined into Flow equivalent mixed empty and void without error.
- Support TypeScript's variance annotation readonly in out, in out without error.

Misc:

experimental.ts_syntax is now always on in https://flow.org/try.

0.229.2

Misc:

Fixed a potential crash under the experimental flag experimental.blocking_worker_communication=false.

0.229.1

Misc:

Bug fixes in preparation of new feature rollout

0.229.0

Likely to cause new Flow errors:

invalid-recursive-exported-annotation will now have error code of recursive-definition.
Previously we would emit confusing errors when you try to use a value that has union types as a type. Now it will consistently emit value-as-type errors. (example).
Unsupported statements like loops within declare module will no longer cause parse errors. Instead, they will be errored on during type checking.
declare export type and declare export interface statements in toplevel are no longer parser errors. Instead, they will now be errored during type checking.
Previous toplevel-library-import errors and unsupported-statement-in-lib errors will all have unsupported-syntax error code now.

New Features:

We introduced experimental.ts_syntax flag that you can set to true. When this flag is on, Flow will no longer complain about certain TypeScript syntax, and will automatically turn them into the closest Flow equivalent types. Initially, we support the following: Readonly, ReadonlyArray, ReadonlyMap, ReadonlySet, NonNullable, and using extends to specify type parameter bounds. In the future, more kinds of types that we currently parse but error on might be added. (e.g. keyof). Please note that the support here is best effort. We make no guarantee that these types have the same semantics as those in TypeScript 100% of the time, but for common cases, it might be good enough for you.

Notable bug fixes:

We now allow the use of saved state for glean indexer, if saved state is enabled in flowoconfig or CLI arguments.
Improved the way we compute contextual hints for spread arguments of calls (e.g. try-Flow).
In declare module, all the values will be auto-exported if we cannot decide whether the module is in CJS or ESM. However, support for declare const and declare let was missing. This has now been fixed.

Misc:

Make both the default value for the .flowconfig casting_syntax option, if not supplied.

0.228.0

Likely to cause new Flow errors:

Some new errors might be raised when using React.cloneElement with polymorphic components. Eg. try-Flow. Note that React.cloneElement is deprecated
Nested declare module is no longer a parser error, but a type checking error
declare module is now explicitly banned in nested scope and outside of library definition files. It already does nothing under those contexts.

New Features:

All kinds of imports are now supported in library definition files
We will no longer require annotations on class properties, if the class properties are already initialized with "annotation-like" expressions. e.g. We can now infer the type directly for properties like prop = (foo: string): number => 3.

Notable bug fixes:

Fixed a bug that causes the first run of flow check to always return 0 errors, when some parts of temporary directory is a symlink.
Hover type on react elements now show the type of the component and props, instead of just React$Element. (e.g. try-Flow)
For missing annotation on exports, we now correctly say some annotation is missing on an identifier instead of array pattern. e.g. for export function f(a) {}, we now say "Missing type annotation at identifier" instead of "Missing type annotation at array pattern".

Parser:

Flow can now parse declare namespace Foo {...} statements. The body of the declare namespace statement shares the same restrictions of declare module.
Invalid exports in declare module are no longer parse errors. They are now type checker errors.

0.227.0

Likely to cause new Flow errors:

Some errors related to empty arrays might have their error locations or codes changed.
Flow will now interpret trivially recursive types, such as type T = T as empty, instead of any. (e.g. try-Flow).
We now error when calling .call() on polymorphic functions where the return type is under-constrained (e.g. try-Flow). To fix add an annotation to the return result.

Parser:

DeclareModule AST will no longer have a kind field

Library Definitions:

Removed redundant cases in Array.from overloads. It might cause some error locations and codes to change.
Add types for the Storage API, available via window.navigator.storage (thanks @ZelJin).

0.226.0

Likely to cause new Flow errors:

$ObjMap and $TupleMap become stricter. The following code now errors: example

IDE:

Linked JSX editing will now only trigger if you edit the opening tag, instead of both the opening and the closing tag.
We now provide IDE services in user-defined library definitions.

Notable bug fixes:

Fixed a bug where recursive types defined across multiple files caused spurious errors and led to unsound types
Add .databases() to IDBFactory (thanks @bobrovnikov)
Mark DOMStringList as iterable, so that it works with Array.from(...) (thanks @bobrovnikov)

0.225.1

Notable bug fixes:

[fix] disable aggressive behavior of linkedEditingRange requests when there are parse errors
[fix] Only print "evaluated" hover type results if the type is of a reasonable size (less than 100 nodes)
[perf] faster LSP commands through skipping redundant parsing
[fix][ide] Find-all-references and rename will no longer return stale results on the file that triggers the request. Instead, we will compute the most up-to-date result at the time of the request for the triggering file.

0.225.0

Notable bug fixes:

We improved the inference support for function rest parameters in a generic function. This example can now type check. You can also extract the parameters of a function excluding the first parameters now: (example)
In hover type, when a name is defined in another file but is imported with a different name, Flow will use the locally aliased name.
fixed a crash happening when a class was defined in a scope where a binding with the same name already existed (try-Flow example)

0.224.0

Likely to cause new Flow errors:

%checks is now deprecated. It will still work as before (for now), but if you enable the deprecated-type Flow lint, you will see additional errors. Instead, use type guards.
$Call, $ObjMap, $ObjMapi, $ObjMapConst are now deprecated. They will still work as before (for now), but if you enable the deprecated-type Flow lint, you will see additional errors. Since Flow 0.209, you can now use builtin types like Parameters, ReturnType, Pick, Omit to easily extract and transform types. For more advanced use cases, you can read the docs of conditional type and mapped type to understand how to write it in equivalent ways.
Ban creation of object literal computed properties with non-literal number keys
Ban accessing objects with non-literal number keys
You can now create and access object keys with int-like number literals, these act as their string equivalents.
Union of exact objects allow computed access as long as one of the members of the union have the property

New Features:

Add support for number literals as keys in object literals, e.g. const obj = {1: true, 2: false}
Support numeric keys in object type annotations. They are equivalent to the string version.

Notable bug fixes:

Removed spurious underconstrained error with mapped type, if the relevant type parameter already has a default. (example)
Support calling Array.from with string input and also mapping function, e.g. Array.from('str', x => x.length)

IDE:

[ide] Go-to-definition for a barrel-exported type name will now jump to the original definition location, instead of incorrectly jumping to itself.

Library Definitions:

Add type stubs for global crypto object, crypto.getRandomValues, and crypto.randomUUID
We removed StatelessFunctionalComponent type from the 'react' module and the React$StatelessFunctionalComponent type. The name might give an incorrect assumption that Flow can actually check that a function component doesn't use stateful hooks or can be called directly. If you have code that still uses this type, we recommend you use the React.AbstractComponent type, since it forces you to not make any assumption of what kind of React component you are dealing with.

0.223.3

Before Flow 0.223, the autoimport system uses the filename as a hint of the name for default export. e.g. if the filename is foo.react.js or foo.js, regardless of whether the default export in that file has a name or not, or even the default name is bar, autoimport will always import it as import foo from 'foo'.

However, in the new system starting from Flow 0.223, we will use the name of the default export as a stronger hint. In a lot of files that just give a default export a random generic name, they are relying on the old system to get reasonable auto imports.

It's not practical to update every Flow codebase to avoid the UX regression, so now auto-import will provide both options.

0.223.2

Notable bug fixes:

Fixed issue with saved state generation used to power auto imports.

0.223.1

Notable bug fixes: Improved precision of server states, that are visible either in vscode status bar, or through the flow status command.

0.223.0

Notable bug fixes:

Mitigates a bug in Linked Editing Range where typing characters quickly on one side would throw off sync on the other side. With this fix, even if some characters are skipped Flow will be able to later recover the matching of open and close tag.
Fixed an issue that will cause some nested conditional types to be evaluated to any. (Example: Issue #9107)

IDE:

Auto-import for default-exported values will prefer the name of the exported value over the name inferred from the module.

Misc:

Error collation is incremental only. The incremental_error_collation flowconfig option is not recognized anymore.

Library Definitions:

Remove status global from dom.js
Delete declared React module. You should always import react with lowercase like require('react') or import React from 'react'

0.222.0

New Features:

Go-to-definition now works for private names
Find references and rename now work for private names

Notable bug fixes:

Documentation will now show up for declare export statements in hover and autocomplete. This means that jsdoc for exports in declare module will finally show up in IDE services.

Library Definitions:

Added USBConnectionEvent
Added SecurityPolicyViolationEvent
Added crypto.randomUUID, http$Server.closeAllConnections, http$Server.closeIdleConnections

0.221.0

Likely to cause new Flow errors:

Changed the signature of React.Fragment to only accept children and the key prop.
Changed the signature of React.useRef to return an opaque type.
Fixed a case where, in some cases, contextually-typed callbacks being passed as props would be typed as any rather than producing underconstrained-implicit-instantiation errors. This will result in new underconstrained errors, but removes a source of untracked unsoundness.
Banned non-tuple arrays from being spread into tuple types. It previously resulted in any without error.

Notable bug fixes:

Improved signature help support for optional chaining calls and intersection types. Now we will show all possible signature possibilities for overloaded functions.

Performance:

Incremental error collation, which helps IDE performance when you have a lot of suppressed errors, is now the default.
Optimized the way we generate global types for normal source files. You might see a reduction in checking times (3-9% as measured internally at Meta). (D51055838 samzhou19815)
Optimized the way we analyze syntactic environment required for type checking. On a large file like this, it improves the checking time from 4s to 2s. (D50850606 samzhou19815)
Optimized substitution performance when conditional type and mapped types are operating on large and deep objects.

Library Definitions:

Updated IntersectionObserver types.
IndexedDB transaction now allows consuming storeNames as DOMStringList.
Added options to IndexedDB transaction.
Added durability to IDBTransaction.

0.220.1

Notable bug fixes:

We will now provide auto type import for default-exported classes.
We no longer suggest defined values when doing auto complete in object keys when we are under a type annotation.

0.220.0

Likely to cause new Flow errors:

Fixed an issue where access of optional tuple element was not considered | void
We rewrote how we generate types for library definitions, which makes the validation of libdef files stricter. You might see more errors as a result of the stricter validation. e.g. all unbound names in libdef files will cause an error, instead of the first reference of the unbound name.

Notable bug fixes:

Fixed a bug in "refactor to react component" code actions. Previously, it would incorrectly replace all jsx children with the newly refactored JSX elements
Fixed signature help when the function called is typed as an alias (e.g. try-Flow)
Fixed some spurious errors in generic calls when the solution involves unions (e.g. try-Flow)
Fixed a bug that caused weird errors when you try to use indexed access in export positions. example

0.219.5

Misc:

Bug fixes in preparation of new feature rollout

0.219.4

Misc:

Bug fixes in preparation of new feature rollout

0.219.3

Misc:

Bug fixes in preparation of new feature rollout

0.219.2

Misc:

Bug fixes in preparation of new feature rollout

0.219.1

Misc:

Bug fixes in preparation of new feature rollout

0.219.0

Likely to cause new Flow errors:

We changed the upper bound of React.Element to be an inexact empty object, effectively making it opaque. React doesn't recommend inspecting children and React.Element, so we make the change to increase friction. See https://react.dev/reference/react/Children#alternatives for alternatives. To keep existing code type safe, you can add a function like the following to explicitly expose the internal of React.Element:

export default function DangerouslyAccessReactElementInternals_DO_NOT_USE_IN_NEW_CODE<
  E: React$ElementType,
  P,
>(e: React$Element<E, P>): {
  +key: React$Key | null,
  +props: P,
  +ref: any,
  +type: C,
} {
  return e;
}

Conditional types will now only distribute over a union if A in A extends B ? C : D is a generic type. This is a behavior change that might cause new errors (example). Previously, we distribute over unions in more cases, which contradicts what we said in the docs, and caused some weird behavior that's fixed in this version.
If an exported class contains unannotated propTypes, Flow will now emit signature-verification-failure.

New Features:

Hovering on enum members in IDE now show documentation if it's available.
Added casting_syntax option which can be colon (current), as, or both. If setting is both, both (x: T) and x as T are allowed. If set to colon or as, only (x: T) and x as T are allowed respectively.

Notable bug fixes:

Previously, when Flow cannot decide which branch to take in a generic conditional type, Flow will produce a abstract type that's unusable almost everywhere. Now Flow will still produce an abstract type, but it will be bounded above by a union type of both branches of conditional type. example.

Misc:

Flow's insert type quickfix in IDE will stop inserting very specific React.Element types. Instead, it will insert general types like React.MixedElement.

Library Definitions:

Add type definition for CanvasRenderingContext2D.roundRect.
Add tagName to DOM element declarations (thanks @aselbie).

0.218.1

New Features:

Error collation during rechecks should become faster thanks to incremental computation. (Available with the flag --incremental-error-collation.)

0.218.0

Likely to cause new Flow errors:

We start to validate variance inside $ReadOnly, which is previously completely unchecked. It also results in Flow complaining about some incorrect bounds. example

New Features:

Hover types will now show a specialized version of the callee expression at a call to a generic or intersection-typed function.

Notable bug fixes:

improve coverage accuracy for the element access portion on element calls, obj[m](arg).

Misc:

We slightly optimized Flow's performance of subtyping check between two union types.

Library Definitions:

Add definitions Array.prototype.findLastIndex(), String.prototype.at(), TypedArray.prototype.at() and Object.hasOwn() (thanks @pascalduez).
Update URL definitions (thanks @pascalduez).
Add typing for cause option in Error constructor.

0.217.2

This is same as 0.217.1, but we have some issue publishing 0.217.1 to npm, so we will publish this new version instead.

0.217.1

Notable bug fixes:

Previously, while global find ref/rename (experimental.global_find_ref=true/experimental.global_rename=true) is ongoing, all other IDE services will be blocked. We fixed it so that other IDE requests can still be served.

0.217.0

Likely to cause new Flow errors:

Flow will now ban variance annotations in function or function type parameters. example
React.Element is now opaque, but we still expose some of its internal structure via the bound on the opaque type. You should not depend on the internal structure. In a future release, we plan to make React.Element truly opaque.
Fixed an issue with implicit instantiation in operations over unions that caused many errors to be missed.
Changed the ref params on forwardRef and useImperativeHandle to be write-only. Patterns that rely on refining these params to an object and reading the ref field are misusing these APIs. example
React.ElementProps is now more precise when applied to AbstractComponents, resulting in the config type of the component rather than mixed. This will cause errors when elements of components with different props are passed into React.Element<> types.

Parser:

We now parse type arguments after typeof annotations. e.g. typeof MyGenericClass<string, number>. Type checking for this syntax is still not supported.

Library Definitions:

Update URLSearchParams definitions (thanks @pascalduez).
Update AbortSignal definitions (thanks @pascalduez).
Add Array copying methods (toReversed, toSorted, toSpliced and with) the $ReadOnlyArray class definitions (thanks @pascalduez).

0.216.1

Notable bug fixes:

We fixed local find references for namespace imported variables. It will no longer error or return empty results.

0.216.0

Likely to cause new Flow errors:

The ability to configure module.system.haste.use_name_reducers=false is removed. For now this option defaults to true and can only be set to true. The option to set the only possible value will be removed in a future release.

Notable bug fixes:

Fixed cases where hovering over a parser error could cause the Flow server to appear to hang.

Library Definitions:

Add USB web API definitions.

0.215.1

Notable bug fixes:

We fixed a bug that causes us to insert extra quote in autocomplete string literal in indexed access types.

0.215.0

Likely to cause new Flow errors:

Improved analysis determining if a function has an implicit void return. As a result, some incorrectly typed code will no longer type check. (example)
Fixed a bug where global assignments in generics wouldn't flag errors. (example)

New features:

Flow now supports contextual typing for class private property assignments. (example)

IDE:

Local find all references will now return a more comprehensive result list.
Go to definition now supports indexed access types.

Library Definitions:

Added inert attribute, showPicker() method and hasPointerCapture() method

0.214.0

Likely to cause new Flow errors:

Flow now has stricter behavior with $Call, $ObjMap, $ObjMapi, $ObjMapConst, $TupleMap. Previously hidden type errors might be revealed now.
You can no longer disable tuple_enhancements, conditional_type, mapped_type, type_guards in flowconfig, since these options have been removed. They were enabled by default since v0.212.0.

IDE:

We now provide a new refactor that extracts JSX elements into a new React component.
Extract to constant refactors now works for nested JSX elements and fragments.

Notable bug fixes:

Added default type arguments for the built-in React$AbstractComponent to match the React.AbstractComponent type alias in our libdefs. [try-flow]

Library Definitions:

Added parseArgs to the Node.js util module library definition

0.213.1

Notable bug fixes:

Fixed a bug where local find references do not return references in jsx props.

0.213.0

Likely to cause new Flow errors:

Builtin idx support is removed. idx functions can now be typed with conditional type and mapped type. The v3 release of idx contains these typing changes.
When using module.system=haste, it will now be an error if a .js.flow file shadows a .js file with a different path prefix.

Notable bug fixes:

Fixed a bug that used to cause spurious internal errors when you have non-binding patterns. example

Parser:

Fixed parsing of multiple as/satisfies casts in a row.

Library Definitions:

Added support for Intl.Locale class

0.212.0

Likely to cause new Flow errors:

Flow is now stricter with regards to type checking of spreads, $NonMaybeType, and $ReadOnly. You might see new errors revealed as a result.
Experimental utility types $Pred and $Refine have been deleted. These are replaced by type guards.

New Features:

Updated Pick and Omit to work on interfaces and instances.
Conditional types, mapped types, type guards, and tuple enhancements are now enabled by default.

Notable bug fixes:

Prevented hover and go-to-definition requests that infinitely loop.

IDE:

For imported names using require, go-to-definition will now jump to the location of the original definition's name.

0.211.1

Notable bug fixes:

Improve fuzzy matching of autoimports

0.211.0

Likely to cause new Flow errors:

We fixed an issue where the intersection of two identical object types are turned into empty in spreads. As a result, more errors might be revealed. example

Notable bug fixes:

We fixed an eager evaluation bug that can lead to some bad interactions between conditional type and mapped type. (example)

Parser:

We fixed the parser size regression since 0.210.0.

Library Definitions:

We rewrote the Omit type implementation. Now it will preserve the optionality and the variance of the input object. example

0.210.2

Notable bug fixes:

Fix a bug where autocompleting a keyword deleted the next 7 characters on the same line
"Fix" sorting of autoimports. An experimental option (autoimports_ranked_by_usage=true) to sort autoimports by how often they are imported was influencing the sorting even when the option was not enabled. This was likely an improvement, but it was a bug to not obey the option.

0.210.1

Misc:

Improvements in preparation of new feature rollout

0.210.0

Likely to cause new Flow errors:

We further strictified React.Element and React refs related typing. You might also see some existing errors related to React.Element and React ref moved around. Example

New Features:

Added Pick, Omit, and Record utility types. [try-flow]

Notable bug fixes:

Fixed a bug where void would appear in autocomplete suggestions more than once

Misc:

Improved keyword autocomplete ordering

Parser:

Library Definitions:

Added missing CSSOM replace and replaceSync
Added arrayBuffers to Node libdefs for process.memoryUsage()'s return type

0.209.1

Misc:

Bug fixes in preparation of new feature rollout

0.209.0

Likely to cause new Flow errors:

We removed support for React PropTypes. All related Flow builtin types like React$PropType$Primitive are deleted. React has deprecated PropTypes since v15.5.
We will now catch errors related to generic indexed access types. Example
Fixed issues with indexed access on interfaces/instances. Access with string literals will now access named properties. example
We now require annotation for computed access of instances, when that access is directly exported.
The return annotation of a function can now reference a parameter name (e.g try-Flow). This may cause new errors if the return type used to reference a name that aliases a parameter name. This will now be captured by the parameter and cause a [value-as-type] error.
We now error when using the name of a variable in its own annotation (e.g. const x: typeof x = ...). An exception is when the name of the variable appears under an object type constructor. For example, the annotation const o: { f: typeof o } = ... is allowed.

Notable bug fixes:

Fix a bug in the previous version that makes declare let/const function scoped. We fixed it to be lexically scoped again.

Library Definitions:

Array.flat will now have better typing support for flattening heterogeneous inputs. e.g. [1, [1,2]].flat() will now correctly have inferred type of Array<number>.
We added support for the following builtin types that are also found in TypeScript:

0.208.1

Notable bug fixes:

Fixed a crash when we are trying to produce a quick fix for missing imports. The autofix for missing imports broken in the previous release would be working again now.

0.208.0

Likely to cause new Flow errors:

Flow will now error on unsupported statements in library definitions, instead of silently ignoring them.
Flow will error more consistently when variables of type any and empty are used as types (e.g. try-Flow)

IDE:

Get definition request will now consistently jump to the name of the definition for ES module exports.
In find local references, if a reference appears in a named import, it will no longer highlight the entire import as reference. Instead, it will only highlight the relevant name.

Notable bug fixes:

Parameter types can now refer to previous parameters in the same parameter list (e.g. function f(x: number, y: typeof x) {})
Report error when a function parameter is redeclared in its body in functions that have default params (e.g. try-Flow)
For all declared names (declare class/var/let/const), we will allow full forward reference. All reads of these values will be considered to be initialized.

Misc:

Changed React.Element to support a second type argument, as supported by React$Element

Library Definitions:

Updated dom libdefs to allow Trusted Type objects

0.207.0

Likely to cause new Flow errors:

When you pass a generic component type to some React utility type like React.ElementConfig<typeof some_generic_component>, we will replace all the type parameters with their defaults or bounds. It can potentially cause downstream errors. (Example)

New Features:

Add built-in (intrisic) element names like div to JSX autocomplete
Mapped types now support operating directly over keys, like TypeScript's Pick and Omit (Example). Mapped type support is still experimental; enable experimental.mapped_type=true to try them.

Notable bug fixes:

Don't check @noflow files in "all" mode
Fixed a source of spurious errors when using refined string keys as computed properties in objects (Example)
Improved Go to Definition on export statements

Library Definitions:

Added rel as an attribute to HTMLFormElement
Added WeakRef class definition
Added ariaHidden DOM property

0.206.0

Likely to cause new Flow errors:

React.Element will now accept a second type argument that specifies the props type. JSX will populate this type argument with actual props passed to component, instead of deriving it from the signature. The change will reveal some broken code, e.g. try-Flow example.
$Shape is deprecated, use Partial instead. Read this blog post for more.
Add a (temporary) option tuple_enhancements to gate our tuple type enhancements (labeled tuple elements, which can be optional or have variance annotations), so we can show a Flow error rather than having users just get syntax errors from various other tooling (e.g. Prettier, ESLint), until all that tooling is updated.
experimental.abstract_locations is removed. Setting this config to true improves Flow's performance. The config defaults to true since 0.139.

New Features:

Allow opaque types with string supertypes to be used as keys in a dictionary, and have that opaque type be preserved when using Object.keys.
To help people familiar with TS, error on and supply a quickfix to the Flow equivalent for the TS types: Readonly, ReadonlyArray, ReadonlyMap, ReadonlySet, NonNullable.

Notable bug fixes:

Error more consistently with [invalid-computed-prop] errors. try-Flow.
Fixed cases where uses of Partial/Required caused errors to be positioned poorly.

Misc:

To improve clarity of error messages we will display mixed instead of a generic type if that generic type has a default bound of mixed. try-Flow.
Object property type incompatibility errors will now contain additional explanations when the error only arises due to object property invariant subtyping.

0.205.1

Fix over-eager IDE autocompletion of types after typing a space.

0.205.0

Likely to cause new Flow errors:

Remove the flow get-imports CLI command.
Some duplicate errors with Arrays are invariantly typed explanations are now correctly removed.

New Features:

Added type-aware autocomplete in IDE for switch cases.
Added signature help to IDE for constructor calls.
Go-to-definition in the IDE can now provide multiple options if the target is defined in multiple locations (for example, in a union).
Add Required and Partial to list of utility types to autocomplete in the IDE.
Added quick fix in the IDE for missing "this." on class member access.
To use the extract to expression/type alias IDE refactors, you no longer need to do exact selections to trigger it. Any selections that include extra whitespaces will be OK too.

Notable bug fixes:

Fix go-to-definition in IDE for constructors (e.g., new Foo()).
When considering a function as a callable object type, we now also check the statics of the function against the object. This may cause new errors. For example, the code in this try-Flow will now error.
We now error more consistently when nested union types are being spread. Before, a bug was causing legitimate errors to not be surfaced. Example with [cannot-spread-inexact] errors: try-Flow. Example with type incompatibility: try-Flow.
Some spurious underconstrained-implicit-instantiation errors might be removed. Previously, some classes that extend any might have caused this issue.
Fix over-eager IDE autocompletion of keywords after typing a space.

Library Definitions:

Added NodeJS process.hrtime.bigint().
Many built-in react components and intrinsics props are now readonly.

0.204.1

Notable bug fixes:

Fix a crash that could happen when canceling a recheck during module resolution

0.204.0

Likely to cause new Flow errors:

Some heap refinements might be invalidated more aggressively, when method calls appear in conditionals. This brings us closer to what happens in the case of the regular function call. (Example)
Always error on the deprecated existential type *, it's just an alias for any

New Features:

Experimental support for Mapped Types like {[k in keyof T]: ?T[k]}! (Example). Look out for a blog post, documentation and support in other tools in the near future. You can try mapped types in your project by setting the experimental.mapped_type=true flag in the options section of your flowconfig.

Notable bug fixes:

Fix autocomplete for super members (e.g., super.|)
Fix hover type of JSX attribute names
Fix showing documentation in signature help for member expressions (e.g., o.m(|))
Fix unused-promise lint false positives for logical and conditional expressions
Fix spurious error in overloaded generic calls when a contextual type is available (Example)
Properly remove the : with inferred predicates in flow-remove-types

Parser:

Error on classes with static prototype fields
Fix missing parse error for class constructors that are async, generators or accessors. The former two were previously type errors rather than syntax errors.

Library Definitions:

Add NodeJS stream promises (thanks @moroine)

0.203.1

Misc:

Add useful information to poorly located error messages. For example, for code like this try-Flow where the error points to a library definition, the error message will show the file that caused the error.

0.203.0

Likely to cause new Flow errors:

$Partial has been renamed to Partial, use the latter instead. Read the blog post for more details.
Support for inference_mode config and related infra such as codemods were removed. You should follow the instructions in the Local Type Inference for Flow blog post to switch to use LTI before upgrading if you haven't already done so.
We replaced the implementation which checks nested $Call and $ObjMap during the check of a polymorphic function call. As a result, more underconstrained errors or type incompatibility errors might appear.
Removed the sharedmemory.log_level flowconfig option and the --sharedmemory-log-level command line option.

IDE:

Implement linked editing for JSX elements.
Added code actions for adding/removing braces from arrow functions.
Adds a quick fix from the deprecated $Partial to Partial.

Notable bug fixes:

Fixed a bug that caused instances of nested utility types to be considered equivalent to empty. e.g. try Flow example.
Make Flow's behavior more consistent by not erroring on .constructor accesses. e.g. try Flow example.
Fixed the file watcher not reacting to any changes on Windows.
Improve the error messages when underconstrained-implicit-instantiation errors are raised in overloaded function calls. e.g. try Flow example
We now propagate any to instance types' type arguments. As a result, some spurious errors under LTI might be fixed (e.g. try Flow example), and some new errors might be revealed.
Fixed the hover type result when an enum object type appears nested in another type structure. e.g. try Flow example.
Fixed the hover type on certain kinds of union types. e.g. try Flow example.
Fixed the hover type on class constructors.

Misc:

Try Flow's behavior can now be configured.

Parser:

Leading (, & and | are now included in intersection and union ranges, respectively.
Improve error recovery for mismatched JSX tags.

Library Definitions:

Change setState in React lib defs to use $ReadOnly<Partial<T>> instead of $Shape<T>.
Make $ReadOnlySet entry type covariant.
Expand the Performance typing with the eventCounts property, according to the W3C standard.
Adjust the type definitions of the mark and measure properties on Performance interface according to the W3C standard.

0.202.1

Misc:

Fix a crash when using experimental.channel_mode=socket on Windows

Parser:

Add "ObjectTypeMappedTypeProperty" node for mapped types
Change "ConditionalType" and "InferType" nodes to "ConditionalTypeAnnotation" and "InferTypeAnnotation"

0.202.0

Likely to cause new Flow errors:

Don't error for tuple element type incompatibility if we have already errored for invalid tuple arity.
Fix a bug that causes certain errors in annotations to be hidden. As a result, more legitimate errors might appear. Example here.

New Features:

Add support for optional tuple elements, e.g. ['s'] and ['s', 1] are both valid values for [foo: string, bar?: number].
Add autocomplete for keywords to the LSP.
Add a code action that inserts jsdoc on a function.
exact_by_default=true is now the default value if the option is omitted from the .flowconfig. Read this blog post for more details.

Notable bug fixes:

Fix LSP rename command to consider aliased named exports.
If a contextually typed parameter has a default, we will filter out null and void from the inferred type of the parameter. This removes spurious errors like this.
Some spurious prop-missing errors might be removed. Example here
Remove some spurious internal errors of Flow. Example here

Misc:

Add option for module.system.haste.module_ref_prefix_LEGACY_INTEROP in the config to allow migration from the automatic_require_default flag and to ES module syntax.

Parser:

Include InterpreterDirective in the AST.
Fix loc/range of postfix arrays that start with a parenthesis, like (A | B)[].
Fix missing error when arrow functions have duplicate params.

0.201.0

Local Type Inference:

Local Type Inference is a rewrite of Flow’s type inference algorithm. This change makes Flow’s inference behavior more reliable and predictable. It replaces a global inference scheme that Flow has relied on since its inception and that has often been the culprit of confusing action-at-a-distance behavior, in exchange for modestly increased type annotations. It is now enabled by default! See this blog post for more details on the new system and steps to upgrade your project.

Likely to cause new Flow errors:

When a variable is initialized at declaration, subsequent assignment in a conditional branch will no longer cause later reads to get a union type. This is already the case if the variable is annotated. As a result of this change, you might get new type errors. Example
When a variable is uninitialized or initialized to null, we use all the assignments to the variables as its initial type. We now no longer consider assignments from a child generic function. As a result, you might get new invalid-declaration errors. Example
Fix object value rest. It will now error if attempting to read a write-only or setter-only property (it reads while copying the properties). Additionally, the resulting rest object will no longer preserve read-only variance (as it is a copy).
The type utility $Partial will now preserve the variance of the input object type's properties. For example, if the the input was a read-only object, now that output will also be a read-only object (with optional properties).
string and symbol are no longer subtypes of an interface
Error on with statements in non-strict mode. It was already a syntax error in strict mode.

New Features:

A native Apple Silicon precompiled binary is now available via the GitHub Releases page. It is not yet available via flow-bin. Internally, we see about 2X faster performance than using Rosetta, and 4X faster performance than on actual Intel Macs.
Add local variable renaming capabilities to the LSP
Add "Find All References" functionality to the LSP for showing local references
JSDoc autocomplete will automatically trigger after typing /**
Add option for automatically closing JSX tags in the IDE
Hovering in the IDE now shows both the unevaluated and evaluated type of the target expression.
Add use_mixed_in_catch_variables config option that makes catch variables default to mixed instead of any
Rename $Partial utility type to Partial. $Partial will continue working as an alias for at least one more version - use that period of time to codemod it to Partial.
Add the Required utility type. It does the opposite of Partial: it makes optional properties required in object types.
Improve error when attempting to call Object.values on a Flow Enum, to point the user to using .members()
Improve error when using number and boolean primitives as a subtype of an interface
Improve error on tuple arity mismatch

Notable bug fixes:

Fix a bug preventing annotation hints from being used in tagged template expressions. This fix may get rid of some underconstrained-implicit-instantiation errors that were otherwise impossible to fix.
Fix error when autocompleting the method stub for a function type without parameter names
Document highlight in the IDE now correctly highlights destructured object keys
Autocomplete can suggest literals when the target type is $Keys

flow-upgrade:

Add codemod to convert usage of the $Shape utility type to Partial. It is possible that this will create type errors that you will have to resolve.
Add codemod to rename $Partial to Partial

Misc:

Remove unused-promise-in-async-scope and unused-promise-in-sync-scope. They can be enabled with unused-promise.
Remove --evaluate-type-destructors flag from type-at-pos command
Add flow save-state --scm flag to ease creation of SCM-aware saved states

Parser:

Fix missing decorators on class fields
Make infer a reserved word in types

0.200.1

Notable bug fixes: We fixed a bug that causes spurious Array is not an array errors in try-flow. Example

0.200.0

Likely to cause new Flow errors:

We now require all generic functions to be fully annotated to prevent generic-escape issues. Some escaped-generic errors will be removed with missing-local-annot errors on return type annotation positions.
Under LTI, we will no longer silently give unannotated parameters in destructuring assignment any type. Instead, they will be properly contextually typed. e.g. try Flow example.
Some additional errors might be revealed after a correctness fix in implicit instantiation. e.g. try Flow example
The exact_by_default option is now required to be set in the [options] section of the .flowconfig. Add either exact_by_default=true or exact_by_default=false to your [options] section. Previously the absence of the option was equivalent to exact_by_default=false. In the future, we will make the absence of the option equivalent to exact_by_default=true. Read more about the option here: https://flow.org/en/docs/config/options/#toc-exact-by-default-boolean. To create a valid new .flowconfig, use flow init.
Changes to type variable unification may cause new errors to surface or old errors to be shifted to new locations.
Fixed a bug where an unannotated parameter does not cause a missing-local-annot error under LTI. e.g. try Flow example
Flow will stop emitting some spurious errors and uncover some hidden bugs under local type inference, as a result of removing a cache that no longer makes sense.
The this type of exported classes' methods are no longer unsoundly typed as any. As a result, more underconstrained implicit instantiation errors might show up in LTI.
Relational comparisons between dates and numbers are no longer allowed.

New Features:

We added flow autofix missing-local-annot command that will try to add annotations to autofix all missing-local-annot errors in a given file.
We now provide autofixes in IDE for all missing-local-annot errors where we can locally infer a type. Previously, this kind of autofix is only provided for unannotated parameters.
Added an unused-promise-in-sync-scope lint which will detect unused promises in sync scopes (like unused-promise-in-async-scope does for async scopes). Enabling unused-promise will enable both of these rules. The individual rules will be removed in a future release.
Pragmas (e.g. // @flow strict, // @jsx custom_jsx_fun) are now supported in try-flow.

IDE:

Added autocomplete for jsdoc comments on functions.
Added autocomplete for $FlowFixMe comments.
Autocomplete will no longer suggest existing type names for type binding identifiers (e.g., in the name of a type alias).

Notable bug fixes:

We will no longer warn about missing-local-annot when there is a proper typing context in super call. e.g. super(x => x) will now pass type checking under LTI if the super constructor call expects such function.
Builtin types like $ReadOnly will no longer shadow your local definitions with the same name.
We will no longer emit spurious errors when we are doing contextual typing while trying to resolve an overload. e.g. try Flow example
We fixed a bug that over-zealously uses contextual type information from sibling nodes to type generic function calls. As a result, examples like the following no longer error: try Flow example
fixed a bug where optional chaining on a value typed as a type application would not filter out the null or undefined value. e.g. try Flow example
Some spurious errors might be removed as a result of Flow doing proper cache invalidation.
Fix sorting of auto-imports with the same similarity score and length.

Parser:

Parse and error on JSX type arguments.
Add custom parse error for abstract classes.
Add custom parse error when attempting to use template literal types in Flow.
Add custom parse errors for TS class visibility modifiers public/private/protected.
Remove function as an alias to Function, and don't allow function as a type name in general. (closes issue #9000).

0.199.1

Notable bug fixes:

Fix crash related to deleted files in Haste

0.199.0

Likely to cause new Flow errors:

Support for spread argument in the builtin special-cased $Compose function is dropped. If you need this, you can write it yourself:

declare function compose<T>(
  fns:...$ReadOnlyArray<T=>T>,
): T;

Flow now consistently disallows implicit coercion of dates to numbers. Prefer explicit conversion with getTime().
When a type argument is inferred from type argument bound or default, and such inference causes error downstream, we will provide better explanations in the error message where the type is coming from. Examples in try Flow.
We fixed a bug under LTI that causes us to incorrectly skip checking for some function expressions Example. Some previously hidden errors might be revealed.
We fixed a bug that causes some empty arrays to be incorrectly inferred as Array<empty> under LTI. e.g. Array.from(nullableStringArray ?? []). As a result, previously hidden errors might be revealed.
Errors for bad == or === comparisons will now consistently show up. Previously, some errors might be hidden when some optimizations are hit, which heavily depends on the implementation details. Example
More missing-local-annot error might be shown in cases when we cannot resolve overload. Example
We fixed a bug where an unannotated parameter does not cause a missing-local-annot error under LTI. e.g.

declare function id<T>(T): T;
id([(item) => 1]); // now errors

New Features:

Improved behavior of string literal autocomplete. Results will be provided regardless of the quote style used, and extra quotes won't be inserted when one is already typed.
Add support for declare let and declare const. These work like declare var, but follow the rules of let and const respectively.

Notable bug fixes:

We will no longer emit escaped-generic errors for predicate function bodies.
$Compose now works under LTI mode.
Remove spurious illegal-this error when a this annotation is used in contextual typing.
Under LTI mode, we will no longer emit spurious incompatibility errors in invalid predicate function like function f({a: b}): boolean %checks { return typeof b === 'string'; }. Instead, you will only get error on this unsupported syntax.
Fix type created by tagged template literals and String.raw. (Closes #7580. Fixes #5705. Fixes #2616.)
We will emit fewer underconstrained-implicit-instantiation errors, when we decide that using type parameter default or bound won't cause downstream errors.
Fixed a bug in LTI where some errors in utility types were not properly shown. Example
Fix IDE services that stopped working while a file contained a setter with the wrong number of parameters.
Fix an issue with lazy mode where deleting a file before the server starts did not check files that were depending on the deleted file.

Misc:

Try Flow is now using the new local type inference algorithm.

Parser:

Parse bigint object keys (but type checking is not support yet).

0.198.2

Notable bug fixes:

Fix a crash on Windows
Fix a bug that could possibly result in a crash

Parser:

Fix bugs concerning await in async formal parameters (including a regression in 0.198.0)

0.198.1

Fix a regression in 0.198.0 that can lead to a crash

0.198.0

Likely to cause new Flow errors:

Implicit coercion of booleans to numbers is consistently disallowed. Previously, booleans could be implicitly coerced to numbers in addition expressions. Prefer explicitly converting booleans to numbers.
Binary arithmetic errors are now reported on the expression rather than the operands, and use error code unsafe-arithmetic.
keyof, undefined, never, unknown, and readonly are now reserved types. If you have a user-defined types of these names, rename them to something else.

New Features:

TypeScript syntax helpers. To help people familiar with TypeScript syntax ramp up on similar Flow syntax:
- Parse and error on keyof type operator, recommending $Keys<...> instead.
- Parse and error on undefined type, recommending void instead.
- Parse and error on never type, recommending empty instead.
- Parse and error on unknown type, recommending mixed instead.
- Parse and error on readonly applied to tuple and array type syntax.
- Parse and error on usage of in/out/in out variance annotations.
- Parse and error on as, as const, and satisfies cast expressions.
Add a codemod to fix underconstrained-implicit-instantiation errors only detectable under LTI. You can run flow codemod annotate-implicit-instantiations --include-lti --ignore-suppressed --write .

Notable bug fixes:

Fix a bug in LTI mode where some errors were hidden in implicit instantiation calls (e.g. Try Flow)
Fix unsound behavior of sentinel refinement with unions of numeric types as tags (e.g. Try Flow)
No annotation is required in LTI for callback parameters when the callee is a refined object member (e.g. Try Flow)

IDE:

When typing in an object literal, entire method signatures from the object type will be suggested.
Method signature autocomplete items are now visually distinguished from other kinds of autocomplete items.
Fix manually triggering autocomplete on the last empty line of a file giving no results.
Members from Object.prototype are no longer included in autocomplete results.

Parser:

Fix parsing of for await (async of ...) (regressed in 0.197.0)
Fix parsing of for (let in x) in non-strict mode
Fix parsing of class fields named static
Fix missing parse errors on keywords containing escape sequences (regressed in 0.180.0)
Fix missing parse error when using await as a parameter in an async arrow function
Fix missing parse error for export { ReservedWord as ... }
Fix incorrect parse error on export * as ReservedWord from ..., which is now allowed
Treat && and || as tokens when parsing types to improve error messages
Parse the argument of the typeof type argument as a value, not a type, as it is a value

Library Definitions:

Add Element.getAttributeNames(): Array<string>

0.197.0

Local Type Inference:

We are releasing a new inference mode called local type inference. You can enable this new mode in your codebase by adding inference_mode=lti to the [options] section in the flowconfig. We will describe more details on this inference mode in an upcoming blog post.

Likely to cause new Flow errors:

Unannotated class properties initialized with null will now errors.
We now require all generic functions to be fully annotated.
bool is a deprecated alias for boolean. Added a deprecated-type-bool lint and flow fix --error-codes deprecated-type codemod to help migrate to boolean. This lint is also included in the deprecated-type lint; if deprecated-type is enabled, you can opt out with deprecated-type-bool=off

New Features:

Tuple improvements:
- $ReadOnly utility makes tuples read-only. E.g. $ReadOnly<[string, number]> is the same as [+a: string, +b: number].
- Tuple elements can now have their variance annotated, when the element is labeled. E.g. type T = [+foo: number, +bar: string] creates a read-only tuple.
- You can now label tuple elements (like TypeScript allows). The labels have no type-checking effect, they just help with self documentation (like indexer labels). When a function argument rest element is a tuple type, the labels work like parameter names in signature help.
- We now parse tuple spread elements, but type checking of this feature is still unsupported.
- These new features are now supported in Flow, but we have not upgraded related tools yet. For example, Prettier won't be able to format your code that uses variance annotations on tuples.
Allow mixed or any as an annotation to a catch parameter (but not anything else)
When typing in a class body, method signatures from super classes and interfaces will be suggested to make overriding/implementing these methods easier.

Notable bug fixes:

Improve the parser's error recovery when in the middle of adding type parameters.
Trigger autocomplete in JSX tags, JSX attribute values, and private class fields.
Improves performance of bigints in big union checks

Misc:

Inferred recursive types are shown as any (instead of V$n) in hover types.

Parser:

symbol is now a reserved type
for (async of []) is now a parse error

Library Definitions:

Updated AbortController and AbortSignal signatures
Actually expose ReactSetStateFunction type introduced in the last release.

0.196.3

Fix a bug that could cause IDE commands to fail while a recheck is ongoing

0.196.2

🎁 This is ~~(hopefully)~~ our ~~last~~second to last release of the year. Happy holidays! 🎁

Improve performance when unions flow to intersections
Fix a crash when using the experimental saved_state.allow_reinit option

0.196.1

Fix some missing autocomplete results (regression in 0.196.0)

0.196.0

Likely to cause new Flow errors:

Instead of mixed, type the result of Object.values and Object.entries on a dictionary to be the dictionary values, and Object.entries keys to behave like Object.keys. Closes #2174, #2221, #4771, #4997, #5838.
Ban array inputs to Object.values and Object.entries.
Function's name and length property are now not writable.
Fix Object.keys, $Values, and $Keys on interfaces with indexers.
Delete $Subtype and $SuperType utilities.
Delete deprecated-utility Flow lint.

New Features:

An annotation is no longer required when exporting + expressions (e.g., export default (3 + 3 : number) can now just be export default 3 + 3;).

Notable bug fixes:

Fix unary and binary arithmetic operations on bigints being exported/imported across modules as any or number.
Fix autocomplete of string literals in bracket syntax, after a quote has already been typed.
Improve sorting of results when autocompleting types.
Boolean CLI flags must now be 1 or true to be truthy. Before, anything other than 0 and false was considered truthy.

Misc:

Autocomplete results are now sorted case-insensitively.
Autocomplete results are now sorted using a fuzzy score, in the same manner as VSCode.

Library Definitions:

Add ReactSetStateFunction type alias in react. This is the type of setVal in const [val, setVal] = React.useState(...).

0.195.2

Misc:

Adds an --experimental.run-post-inference-implicit-instantiation true/false CLI flag to override the respective experimental flowconfig option.

0.195.1

Misc:

Improved performance for the experimental post-inference implicit instantiation checks.

0.195.0

Add a quick fix for the unused-promise-in-async-scope lint which will insert void.
Add symbol and bigint to $NotNullOrVoid
Add findLast to Array definition

0.194.0

Likely to cause new Flow errors:

Casting an enum to empty will now properly error, e.g. (E.A: empty).

New Features:

Added a quick fix for the unused-promise-in-async-scope lint which will insert await.
Added support for a number of additional bigint features. These include allowing comparison (<, >, <=, >=), update (++, --) and arithmetic operators (+, -, ~, *) to be used with bigints.
Added sketchy-null-bigint lint to match the behavior of the other sketchy-null-* lints for bigints.
Added Flow Enums support for declare enum and declare export enum. Keep in mind that users need to have enabled Flow Enums if you wish to create library definitions that use them.

Parser:

Switch to dune to build and install the flow_parser OPAM package. Some modules in src/parser/ like Comment_utils are newly available.
Decrease the size of flow_parser.js by roughly 25%, to 768kb.
Fix a bug where declare opaque type was parsed as an OpaqueType when interned comments are disabled.

0.193.0

New Features:

Add support for bigint. This includes the bigint type itself, subtyping with literals, libdefs, typeof, and sentinel refinement. For now, literal types are limited to 64 bit ints. Later changes may lift this restriction as well as implement support for operators.

Notable bug fixes:

Fix a few bugs related to the Haste package resolver. The Haste resolver falls back to the Node resolver when resolving packages. It now obeys the module.system.node.root_relative_dirname setting when doing so. Also, scoped package names are now supported in the Haste module resolver.
Allow private readonly properties to be written to in constructors, just like regular readonly properties.

Misc:

Update team members in CONTRIBUTING.md.
When using module.system=haste, importing packages in node_modules folders now obeys Node's resolution algorithm, rather than being able to be required from anywhere in the project.

Library Definitions:

Add a default generic value for getOwnPropertyDescriptor.

0.192.0

Notable changes:

The exact_empty_objects flag and the annotate_empty_object codemod are now deleted, See https://medium.com/flow-type/improved-handling-of-the-empty-object-in-flow-ead91887e40c for more details.
We have announced in a previous blog post that we are working on a new inference mode in Flow. Now the inference mode is mostly working, but there might still be some bugs remaining. You can try it out by adding inference_mode=experimental.lti to your flowconfig.

Likely to cause new Flow errors:

Unknown computed properties assignments are now completely banned. Previously, they were still allowed if the object is inexact.
cannot-resolve-name errors now are emitted for every use of an undefined variable, not just the first.

New Features:

Added an experimental flow lint for unused promises in async scope. You can enable it by adding unused-promise-in-async-scope in the lint section of your flowconfig.

Misc:

annotate-use-state and annotate-use-callback codemod commands have been merged into annotate-react-hooks4
Added an experimental "scm" saved state fetcher, which selects a saved state based on your currently source control revision.

Library Definitions:

Added support for Array.prototype.at
Fixed WeakSet type parameter bound to be interface {}

0.191.0

Likely to cause new Flow errors:

The exact_empty_objects option is now enabled by default. Details about this change and how to update your codebase are covered on our blog.
The typeof X type annotation now respects the initialization state of X: if X hasn't been declared it will be an error to get its type.

Notable changes:

The enforce_local_inference_annotations config option was removed. To migrate your codebase, run the following codemod to add the required annotations: flow codemod annotate-lti. More details on the changes are explained in the Requiring More Annotations to Functions and Classes in Flow blog post.
Literals can be suggested by autocomplete in more cases
Fixed autocompleting specifiers in an import statement

0.190.1

Fix a bug on Windows where the server would exit unexpectedly after hg update
Performance improvements

0.190.0

Note: Due to large number of likely new Flow errors that will be introduced in the next release, we decided to create an extra release to ease the upgrade process. We announced in the recent blog post that we would remove the enforce_local_inference_annotations option in flowconfigs, as it became the default in v0.189.0. This change will be delayed to v0.191.0 instead.

Likely to cause new Flow errors:

Flow will now error on indexed write to exact objects without indexed type.
Flow now requires that the target of extends clauses in class definitions are either variables, casts, or member expressions, rather than allowing arbitrary expressions.
We fixed a bug in which type arguments were not passed to the type parameters of static callable methods.
We unblocked constraints involving any and type applications like Promise<T>, which may cause new downstream errors to be discovered.

Notable bug fixes:

Fix a bug that can cause the Flow server to unnecessarily restart when Watchman restarts

0.189.0

Likely to cause new Flow errors:

enforce_local_inference_annotations is now on by default. To migrate your codebase run the following codemod to add the required annotations: flow codemod annotate-lti. More details on the changes are explained in the Requiring More Annotations to Functions and Classes in Flow blog post.

New Features:

Class members can be suggested by autocomplete even if this is not typed explicitly.

Notable bug fixes:

We will now consistently emit object-this-reference errors. Previously these errors were missing when the object is an argument of Object.freeze(), Object.defineProperties() or Object.create().
Fix crash when walking directories that can't be accessed.
Fixed a potential crash when two type identifiers are declared in the same block.

Library Definitions:

Improve fs.readFile type definition when the encoding config option is supplied.

0.188.2

Bug fixes:

Fix crash during incremental recheck

Performance:

Improve performance of auto-imports index update for large codebases

0.188.1

Bug fixes:

Fix a crash when querying hg

Misc:

We renamed the codemod command flow codemod annotate-lti to flow codemod annotate-functions-and-classes to clarify that this is not the only codemod necessary for enabling LTI eventually.

0.188.0

Likely to cause new Flow errors:

Previously we supported a weird JS runtime rule for constructor returns when you call new SomeClass:

If the returned value is primitive, then it's ignored and the instance will be returned instead.
If the returned value is object like, then the return object will be the eval result of new SomeClass. This is confusing, and we dropped support for this feature in this release. Now we enforce that all class constructors must return void.

Fixed a bug preventing refinements from being invalidated in inline && expressions: examples like typeof x === 'number' && reassignXToString() && (x: number) were passing even if reassignXToString() assigned a a string value to x.

New Features:

flow codemod annotate-lti is now provided to help prepare your codebase to enable the enforce_local_inference_annotations=true flag. In order to eventually enable local type inference, we will require additional annotations in places we can't contextually type your code. You can enable the annotation enforcement now with enforce_local_inference_annotations=true in flowconfig. This flag enables the behaviors of all the following experimental flags, which will be deleted in the next release:

experimental.enforce_local_inference_annotations=true
experimental.enforce_this_annotations=true
experimental.enforce_class_annotations=true

To migrate your codebase, you can run the following codemod to prepare your codebase for the new requirement: flow codemod annotate-lti. Check out an upcoming blog post for more details.

Misc:

Remove the file_watcher.watchman.survive_restarts flowconfig option, which was deprecated in 0.158.0. We will always try to survive Watchman restarts.
Remove the experimental.refactor flowconfig option. It has been true by default since v0.158.0.

Library Definitions:

Updates the typing of 'fs.promises.readdir()' to reflect the possibility of passing 'withFileTypes: true' and receiving an array of Dirent objects

0.187.1

Fix a crash introduced in 0.187.0 when the file walker fails to open a directory

0.187.0

Likely to cause new Flow errors:

JSX expressions in a file with jsx pragma of member expressions (e.g. @jsx Foo.bar) will no longer be incorrectly considered as unreachable. This might uncover previously hidden errors.
Functions statics (i.e. assigning properties to functions) are safer, but also a bit more restricted. Function statics should be assigned in the same statement list as the function (i.e. not conditionally).

New Features:

When autocompleting in the middle of a word, autocomplete now inserts the suggestion, instead of replacing the entire word. For example, if you are completing in foo|Baz (cursor at |) and choose fooBar, you'll now get fooBarBaz instead of fooBar. In VS Code, you can press shift-enter to replace instead of insert.

Bug fixes:

Fix Go to Definition on certain kinds of import statements
Improve location of error references pointing at imported namespaces (D38978756 mroch)
Fix some missing references in CLI errors (D38977164 mroch)
Improve location of error messages referring to declare module exports (D38977152 mroch)
Change flow get-def --json to write {"error": message} to stdout on error, instead of writing non-JSON to stderr
Fix a garbage collector bug that rarely could lead to crashes

Library Definitions:

Allow access of arbitrary properties from import.meta. They are typed as mixed
Add support for MediaSourceHandle

0.186.0

Notable breaking changes:

Support for inference_mode=classic has been removed. The default is now inference_mode=constrain_writes. Please refer to the blog post for migration paths. Additionally we have removed all codemod commands that depend on the classic inference mode. This includes the annotate-declarations, rename-redefinitions and annotate-escaped-generics codemods.

Likely to cause new Flow errors:

Exported functions can only have properties assigned in the same scope as the function.
Improve behavior of index property access with union types (try-Flow example). May cause new errors in interaction with computed properties.
Referencing this in object getters and setters are now banned. Referencing this in object methods are already banned, and this change makes Flow's behavior more consistent.

Notable bug fixes:

Fixed a regression in destructuring checking with computed properties (issue).

Library Definitions:

Added PerformanceServerTiming definition and the serverTiming attribute to PerformanceResourceTiming.

0.185.2

New Features:

Flow now supports the @jsxRuntime pragma in the top docblock of each JS module. Allowed values are classic and automatic.

0.185.1

Misc:

Improve the experimental mode to rank autoimport suggestions by how often they're imported (autoimports_ranked_by_usage=true)

0.185.0

Likely to cause new Flow errors:

Unbound names in jsx-pragma when the jsx-pragma is not a simple identifier will now consistently point to the jsx pragma.
Errors previously hidden by method-unbinding will reappear.
Fix a bug that makes assignments in loop guards like x = some expression containing x have a type that's effectively any. If you turned on inference_mode=constrain_writes since the last release, there will be new errors.
Remove esproposal_export_star_as as an option and make this the default as it made it into the JS standard in 2018.
Additional value-as-type errors might be emitted, as a result of existing importing errors.
Object rest on instances no longer results in an unsealed object (that can have properties added after the fact)

New Features:

inference_mode=constrain_writes will be the default. inference_mode=classic, which is the current default, will be removed in the next release. Please refer to the blog post for migration paths.

Misc:

Fix flow-remove-types/jest transformer to be compatible with Jest 28. (Thanks @carloscuesta)
Add codemod to remove annotations in destructuring - which we banned in 0.176

Library Definitions:

Add missing .x and .y properties to DOMRect. (Thanks @pmer)

0.184.0

Upcoming Breaking Changes:

New Inference Mode:

We are releasing a new inference mode that marks significant progress on our way to local type inference. You can enable this new mode in your codebase by adding inference_mode=constrain_writes to the [options] section in the flowconfig.

We will make inference_mode default to constrain_writes in v0.185, and remove the classic option in v0.186. To migrate your codebase, you can run the following codemods to prepare your codebase for the new inference mode:
```
flow codemod rename-redefinitions
flow codemod annotate-declarations
```
We will describe more details on this inference mode in an upcoming blog post.
Additional Annotation Requirements:

In order to eventually enable local type inference, we will require additional annotations in places we can't contextually type your code. You can enable the annotation enforcement now with the following flags in flowconfig:
```
experimental.enforce_local_inference_annotations=true
experimental.enforce_this_annotations=true
experimental.enforce_class_annotations=true
```
To migrate your codebase, you can run the following codemod to prepare your codebase for the new requirement: flow codemod annotate-lti-experimental

Likely to cause new Flow errors:

Object.create now results in a sealed object. Arbitrary properties can no longer be read/written from/to it. You can use the __proto__ property in object annotations to create an annotation if you need to.

Notable bug fixes:

Fix spurious object subtyping errors when an object literal is passed into rest arguments with tuple type

function emit(...args: [{+bar: string}]): void {}
emit({ bar: "" }); // no longer emits incompatible-variance

Fix a bug on Windows that could lead to a crash if files are changed faster than they can be rechecked

flow-upgrade:

Add a codemod to collapse multiline object initialization into one object literal - useful for enabling the exact_empty_objects flowconfig option

Library Definitions:

Add DOMTokenList index signature

0.183.1

Fix a regression in 0.183.0 on Windows where inconsistent handling of slashes within paths led to mismatches like "duplicate provider" errors

0.183.0

Likely to cause new Flow errors:

We now emit new errors in places where an object literal has literal types, but doesn't have an annotation. You can usually fix these errors by adding annotations to object literals.

New Features:

Allow export of empty object without annotation when exact_empty_objects is enabled.
Improve annotate useState codemod to support null/undefined/empty-array arguments.

Notable bug fixes:

Allow flowconfigs with blank lines before the first config section.
Fix location of error message about legacy octal number types.
Fix inconsistent blank lines around auto-imports.
Fix bug where Watchman events might be ignored on Windows.

Parser:

Fix parser exception on negative binary and octal number types that contain whitespace after the minus sign.

Library Definitions:

Improved fs.readdir and fs.readdirSync to allow better overload resolution.
Add String.prototype.replaceAll() (Thanks @dav-s).

0.182.0

Likely to cause new Flow errors:

var re-declarations are now banned. You should use let or const instead.
Flow will now emit escaped-generic errors when the return type of a generic predicate function contains type parameters defined by the function.

Notable bug fixes:

Improve ordering of autoimport suggestions, so that globals are suggested above named imports.

Misc:

Errors related to class expressions might have new locations.

0.181.2

Fix the rendering of classes in autocomplete results
Fix the ordering of class methods in autocomplete results
Improve ability to recover from Watchman restarting (the fix in 0.181.0 was incomplete)
Fix a bug where warnings would be incorrectly reported when using saved state, and could cause a crash.
Add --saved-state-verify flag to check that a saved state matches what is expected to be on disk.

0.181.1

Notable bug fixes:

The JS build for try-flow is now fixed. It's recently broken since the 0.181.0 release.

0.181.0

Notable bug fixes:

Fix unsound spreading of empty array literals (example)
Fix a race condition that could cause individual requests from the IDE to hang indefinitely
Improve ability to recover from Watchman restarting

Parser:

Fix parsing of invalid UTF-8 to raise the correct "Malformed unicode" parse error
Fix line numbers when regex character classes contain an (invalid) newline, which could've led to a crash

Library Definitions:

Add AudioContext constructor options
Update Node's v8 module

0.180.1

Notable bug fixes: Reverted changes that cause Flow to generate a corrupted saved-state when libdef changes.

0.180.0

Likely to cause new Flow errors:

Generator functions without return type annotations now default the type of the next parameter to void. If a non-void next type is desired for a function, it will have to be annotated.
When experimental.enforce_local_inference_annotations is enabled, Flow now requires annotations on functions returned from async or generator functions.
Return statements that return objects no longer widen the object type to match the return annotation, if present. This may lead to additional errors, which can be addressed by adding annotations onto variables that are returned.

New Features:

Optimize initialization from saved state.
When experimental.enforce_local_inference_annotations is enabled, Flow now has a code action to auto-annotate parameters of functions that have missing-local-annot errors.

Notable bug fixes:

Fix broken IDE services when the file contains certain invalid type annotations, import type * ..., export async ... statements or nameless named exports.
Fix some potential deadlocks on Windows.

Misc:

Add --no-enums flag to flow ast.

Parser:

Fix a runtime exception when parsing regexes via the JS version of the parser.
Fix crash when the input is a lone quote.
Fix missing syntax errors when functions with non-simple params contain "use strict"

Library Definitions:

Add type signature for Navigator.canShare().

0.179.0

Likely to cause new Flow errors:

Improve the type checking of function parameters that use destructuring, have an annotation, and also have a default value.

Notable bug fixes:

Fix IDE services in the presence of certain types of parse errors.
Fix crash when encountering an unexpected \/. This could have broken IDE integrations while typing.
Fix crash with invalid for-await loops.

Parser:

Add parser support for ES2022 d RegExp flag (see RegExp Match Indices).

Library Definitions:

Add void to the React$Node type.

0.178.1

Notable Bug Fixes:

[fix] Fix a crash during incremental recheck when an empty string import (e.g. import '') is added.
[fix] Fix a stackoverflow caused by extremely large objects
[fix] Fix an incremental update issue where Flow would sometimes resolve requires to stale Haste modules which have no provider files

0.178.0

Likely to cause new Flow errors:

Improve type incompatibility error messages on default function parameters
Invalidate refinements on calls of require, since requiring a module can have side effects

New Features:

Add types for React 18 hooks: useId, useInsertionEffect and useSyncExternalStore

Notable bug fixes:

x instanceof A when x is typed as any refines it to A instead of empty
Fix an issue with [include] entries containing both globs and relative paths on Windows
Fix crash when declaring a destructured binding in a library definition
Fix missing autocomplete results for JSX component properties when the component's type is recursive
Fix analysis of unreachable += expressions
Fix incremental typechecking bug when adding files in Haste mode

Parser:

Fix parser exception when parsing export interface ... or export enum ...

0.177.0

Likely to cause new Flow errors:

Fix a bug where refinements should be invalidated when going through multiple control flow branches (thanks @gnprice)
Make catch parameters explicitly any typed
Ban object spreads of numbers and strings
Fix a bug that makes Flow consider some function names to have the empty type
Numbers, booleans, and enums are no longer subtypes of the empty interface interface {}
Previously incorrectly missing errors might appear in expressions of the form name === 'literal' and name.prop === 'literal' complaining that literal is incompatible with the type of name or name.prop.
Update input type of Object.{values,entries} to be interface {} rather than $NotNullOrVoid; number and boolean inputs will now error

New Features:

Add codemod to add explicit type arguments to useState when it is called with an empty object. Use with flow codemod annotate-use-state
Continue type checking files with indeterminate module types.

Notable bug fixes:

Do not add further errors when we already errored on the illegal reassignment.
Flow now checks whether literal is a subtype of expr in expr === literal, regardless of whether expr === literal can be applied as a refinement
Fix a crash when deleting a file or saving a file with parse errors in certain circumstances
Fix a rare crash when files are deleted during rechecks
Fix a parser bug that allowed missing commas between object properties (regression in 0.175.0)

Parser:

Improve error recovery when in the middle of adding a property to an object type literal

Library Definitions:

Add NotificationEvent to service workers API

0.176.3

Notable bug fixes:

The Haste module resolver now chooses Haste modules before node_modules, to match what Metro does. That is, if you use module.system=haste and have both a @providesModule foo file (or use "haste paths" to derive the module name from its filename) and a node_modules/foo folder, require('foo') will now resolve to the Haste module, even when node_modules/foo is in that file's parent directories. This is also a significant perf improvement because resolving a Haste module is much cheaper than searching for node_modules.
Fix several bugs responsible for crashes
Fix bugs responsible for some LSP "server is stopped" errors

0.176.2

Revert changes that are causing bugs in non-lazy mode.

0.176.1

Fix crash when comparing abstract and concrete locations when simplifying normalized types.

0.176.0

Likely to cause new Flow errors:

Banned usage of new on functions. Move usages of this pattern to use ES6 classes instead. If the pattern exists in third-party code that cannot be changed directly, you can use a declaration file to type the legacy pattern with a declare class.
Error on type annotations nested inside of destructuring - these were always invalid, we just ignored them before

Notable changes:

Removed special support for React.createClass from Flow. It is now just typed as any. Migrate any components using it to class components or function components.
Added the module.missing_module_generators option, which can be used (multiple times) to specify 'regex' -> 'command' pairs. When a module is missing, if its name matches one of the regexes, we will add a suggestion in the error message to run command to generate it (and resolve the error).
Fixed bugs responsible for some LSP "server is stopped" errors
Fixed a bug that could cause the server to become unresponsive on Windows

0.175.1

Bug fixes:

Fix an incremental bug which would sometimes cause Flow to choose the wrong provider for a haste module, most commonly when one of those providers is a .js.flow file.
Fix ability of flow stop to kill stuck servers
Fix rare crashes when processes exit unexpectedly

0.175.0

Notable changes:

Improve error messages when using a type as a value
Improve error messages in for loops
Correctly determine the scope of default expressions in function params
Add FLOW_CONFIG_NAME env as alternative to passing --flowconfig-name
Add file_watcher.mergebase_with.{git,hg} configs to support projects accessible via multiple VCSs (e.g. git mirrors of hg repositories)

IDE integration:

Surface deprecated autocompletion results so they appear with a strikethrough in VS Code
Fix a bug where changing libdefs, package.json files or the .flowconfig multiple times will cause the IDE to report "server is stopped".
Fix a bug where the IDE is unable to start the server if it is downgraded
Improve suggestions when autocompleting keys in an object literal
Fix an exception when autocompleting a result that would add to an import to an existing require()

flow-remove-types:

Fix handling of this param with trailing commas

0.174.1

Fix crash when Flow receives duplicate file deletion notifications introduced in v0.174.

0.174.0

Likely to cause new Flow errors:

Flow now detects sketchy null errors that arise via optional chaining, and which involve opaque types, type aliases, or generics.
Fixed an issue that caused us to previously miss some errors when the type involved was an intersection.

New Features:

Access from a union of exact objects, on a property that is on one but not all of the objects, now results in the type of that property or void, instead of an error.
Add type checking support for logical assignment operators.

Notable bug fixes:

Fix a crash when using the default file watcher (dfind) on Windows if an [include] directory doesn't exist.
Do not attempt to unify the type of a declare function that comes after a function declaration of the same name (try-Flow).
Improve wording of class-extends errors (try-Flow).
Don't report this-in-exported-function for functions with a this parameter.

Misc:

Added --list-files flag to status and check commands. This will dump a list of files that contain errors instead of the error messages.
The default file watcher (dfind) now logs to a .dfind_log file alongside the .log and .monitor_log files.
Changed the way Flow computes coverage for type parameters with uncovered upper bounds. Previously, a type parameter was always considered covered when it appeared in a function signature, class declaration, or type alias, even if the type parameter's upper bound was any. With this change, such type parameters are now considered to be uncovered. This results in Flow now computing a lower coverage percentage on modules where this pattern is present or that import types from other modules that use this pattern.

0.173.0

Likely to cause new Flow errors:

Modified the behavior of instanceof checks and other refinements to more consistently detect when impossible or invalid refinements are made. This will result in new errors in such cases.

New Features:

Improved error messages by providing a suggested property when properties are missing in a conditional context.

Notable bug fixes:

Fix "matching property" checks with generic inputs (try-Flow). Thanks @gnprice for bringing up this issue and exploring potential causes!
Fixed a bug where the use of certain utility types in polymorphic functions caused errors to not be reported (e.g. try-Flow)
Fix a bug where the [method-unbinding] error was reported twice

Misc:

OCaml version 4.12.1 is required to build Flow
React default exports are now inexact, as technically this can include unknown properties (experimental features)

Parser:

Fixed a bug where type parameter instantiations that start with < (e.g., f<<T>()=>S>()) would be incorrectly parsed as the left shift operator.

Library Definitions:

Add bindings for NodeJS util.types

0.172.0

Bug Fixes:

Exact objects are treated as truthy in logical expressions
Add support for operations on $Values<> types in an export position (e.g. Try Flow)
Respect the order of module.file_ext when resolving imports. Before, they were tried alphabetically.

Misc:

Change signature-verification-failure from a lint to a normal error. It is no longer valid in the [lints] section of .flowconfig.
Improve performance of IDE commands when Flow is busy typechecking

0.171.0

Likely to cause new Flow errors:

Disallow duplicate member names in classes.
Error if an interface is imported as a value.
The experimental.new_merge option becomes true by default. This is an internal change in the way file signatures are computed, but may cause new [invalid-exported-annotation] errors when a file exports types that include certain kinds of complex operations, such as projections over complex utility types like $ObjMap. These errors can be fixed by replacing these complex types with simpler ones. In certain cases, the newly added $KeyMirror and $ObjMapConst types (that are exempt of this restriction) can be used instead.

Notable bug fixes:

Fix Watchman file watcher on Windows when an include path doesn't exist.
Fix a parse error when a newline appears after a class method definition's name.

Parser:

Add support for parsing logical assignment operators (thanks @strub).
Fix compliance with estree: export * as x from 'y' is now an ExportAllDeclaration with an exported key.
Permanently enable parsing of nullish coalescing, and remove the esproposal_nullish_coalescing option.
Permanently enable parsing of optional chaining, and remove the esproposal_optional_chaining option.
Permanently enable parsing of class static fields, and remove the esproposal_class_static_fields option.
Permanently enable parsing of class instance fields, and remove the esproposal_class_instance_fields option.

Library Definitions:

Make some options parameters in node's fsPromises optional.

0.170.0

Another light release due to the new year. Changes are mainly internal -- refactoring or progress towards future features.

Notable breaking changes:

Disallow duplicate member names in classes

0.169.0

This release is light on changes due to the holidays. Happy New Year!

Improve performance

0.168.0

Notable bug fixes:

Improve performance of IDE requests when using lazy mode and saved state
Improve performance of checking union types by optimizing enums more aggressively

0.167.1

Notable bug fixes:

Fix crash when moving a module introduced in v0.167.0

0.167.0

Notable bug fixes:

Error on invalid @flow modes

Misc:

Remove the --profile, --json and --pretty flags from flow force-recheck
Reduce usage of shared heap space

Library Definitions:

Add support for Node inspector
Improve Document definition
Add document.contentType
Add 'navigate' as valid request ModeType (Thanks @comp615)

0.166.1

Fix a crash introduced in 0.166.0

0.166.0

Improve recheck performance on large projects
No longer support flow check-contents on non-Flow files by default; pass --all to force it.
No longer attempt to provide LSP documentHighlight on non-Flow files.
Remove legacy "weak" mode. The --weak CLI flag and weak=true flowconfig option have been removed.

0.165.1

Notable bug fixes:

Improve performance of signature help
Fix a bug that caused rechecks to be unnecessarily invalidated and restarted

0.165.0

Likely to cause new Flow errors:

The Flow parser now requires that variables declared using the declare var syntax be annotated with a type. declare var x; is now illegal.

Notable bug fixes:

No longer attempt to provide autocomplete or flow autocomplete on non-Flow files.
Fix a crash when deleting a file in lazy mode
Fix a bug that prevented some code actions from adding type imports for export default class definitions

0.164.0

New Features:

exports/module.exports can no longer be read from in a module (e.g. exports.foo) to eliminate this usage of unsealed objects - you can refactor your code to directly reference foo instead, or just use ES modules (e.g. export const foo = ...)
Improved autocomplete for the targets of typeof type annotations.
Support for finding references and renaming has been removed. While these features often worked well, they also caused extremely bad performance and crashes. We hope to reimplement these features as soon as we can.
When displaying types, always use indexed access types for $ElementType and $PropertyType utilities.
Remove indexed_access option, it's always enabled now (was on by default before if the option was omitted).

Notable bug fixes:

Fix an issue where the refactor code-action would hang on large inputs.
The exports of a file with no value exports are typed as an exact empty object rather than an unsealed object. Common errors which resulted from this are accidentally doing import typeof Foo rather than import type {Foo}
Fix error causing LSP requests to fail.
Fix calling watchman on Windows.
Fix soundness issue when comparing polymorphic types (example try-Flow).
Fix handling of duplicate bindings in signature builder.
Improve error messages and type-at-pos results around $ExactT<...> type annotations.
Fix referencing this in a JSX title member.
flow coverage now returns 0% for non-@flow files; pass --all to also check non-@flow files.
No longer attempt to provide definition or flow get-def on non-Flow files.
No longer attempt to provide hover or flow type-at-pos on non-Flow files
Update to latest LSP 3.17 beta definition of CompletionItemLabelDetail

Library Definitions:

Fix navigator.clipboard.write type and add ClipboardItem (thanks @Egrodo).
Add node.js fs.copyFile overload to support the case where the mode param is not specified (thanks @Brianzchen).

Misc:

Removed the flow suggest command.
Improve codemod CLI error handling (Thanks @gnprice).
Changed scheduling of LSP commands in parallel with typechecking, to prioritize commands.
Removed flow find-refs command.
Removed the --expand-type-aliases flag from all Flow commands, it has caused several performance and non-termination issues without providing much value.
Improved codebase quality (thanks @gnprice).

Parser:

Invalid typeof type annotations, where the argument to typeof is a not a variable or member expression, are now parse errors rather than type errors.

0.163.0

New Features:

The deprecated * type is now an alias to any. You can use flow codemod replace-existentials, which is available in previous versions of Flow, to upgrade your codebase to not use *. This codemod command will not be available in the current or future versions of Flow.
Add (experimental) codemod to annotate variables that cause constrained write errors in future Flow versions.
Add (experimental) codemod rename variable names that are reused with different types.
Add --include-comments and --include-locs flags to the flow ast command.
Add $ObjMapConst<O, T> builtin type, alias for $ObjMap<O, () => T>.

Notable bug fixes:

Fix bug in $KeyMirror so that it respects the optionality of its arguments.
Type JSON as object rather than a class. This prevents code such as new JSON() which is a runtime error.

Library Definitions:

Add InputEvent.dataTransfer
Declare AggregateError class - fixes #8764 (thanks @lachlanhunt).
Improve UIEvent, InputEvent and KeyboardEvent types.

Misc:

Fix typo in CONTRIBUTING.md (thanks @jereef).
Clarify comments for ConcretizeTypeAppsT logic after Subtyping_kit refactor (thanks gnprice).

Parser:

Fix a bug that allowed comments after the # in private identifiers.

0.162.2

Fixes a bug with experimental.prioritize_dependency_checks

0.162.1

Fix a bug where errors in node_modules were shown in lazy mode. It is now consistent with non-lazy mode.

0.162.0

Likely to cause new Flow errors:

As part of our work on Local Type Inference, unannotated variables now require a non-null value to be assigned to them at some point in the program. If a variable is intentionally only null, it can be annotated with null (let x: null = null); if only undefined, then it can be annotated with void (let x: void;).
Function and class names can no longer be reassigned, even when not exported
Fix positioning of [missing-type-arg] errors

New Features:

Add $Partial utility type. This utility converts all of an object's or interface's named fields to be optional, while maintaining all the object's other properties (exactness, sealedness, etc).

Notable bug fixes:

Improve performance of strict_es6_import_export
Fix an infinite recheck loop when deleting a file when using saved state
Fix a bug where the server would stop after running a CLI command if the server was starting with --autostop

Misc:

Type import.meta as object
Fix jsdoc support on imported declare function
Make string literal autocomplete obey format.single_quotes setting
Improve reliability of "refactor" code actions

0.161.1

Fixes a bug with experimental.prioritize_dependency_checks

0.161.0

Likely to cause new Flow errors:

Only consider variables as "constlike" if their assignments are within their declarations. This fixes cases of unsoundness. example. New Features:
Add hierarchical document symbol support to the LSP. Improves VS Code's Outline pane, breadcrumbs and symbol search.
Autocomplete now includes primitive types like string and number, as well as utility types like $Call and $Keys.

Notable changes:

Significantly improved overall performance.

Notable bug fixes:

Unknown or redundant settings in the flowconfig [lints] section are now ignored when passing --ignore-version.
Improve results of type-at-pos on $ReadOnly types.

flow-remove-types:

Added a Jest transformer, flow-remove-types/jest

0.160.2

The fix in 0.160.1 didn't fully fix the regression in 0.160.0. It, and the original "fix" from 0.160.0 are reverted.

0.160.1

Fix a regression in 0.160.0 that caused suppressions to be ignored upon editing files in some cases when using lazy mode.

0.160.0

New Features:

The ide, fs and watchman lazy modes have been merged into a single mode. The new mode is now SCM-aware with or without Watchman; when the server starts, it queries Git or Mercurial and checks all existing local changes (changes since the mergebase).

The file_watcher.watchman.mergebase_with config option has been renamed to file_watcher.mergebase_with; set this to the name of your default remote branch (e.g. origin/main).

Flowconfig setting migration:
- lazy_mode=watchman -> lazy_mode=true, file_watcher=watchman
- lazy_mode=fs -> lazy_mode=true
- lazy_mode=ide -> lazy_mode=true
- lazy_mode=none -> lazy_mode=false
Added parser support for import.meta
Added type checking support for new.target and import.meta as mixed

Library Definitions:

Add type for queueMicrotask

Notable bug fixes:

Improve codemod performance on large codebases
Fix a bug that could cause "unused suppression" warnings to appear incorrectly when using lazy mode

0.159.0

New Features:

Add support for git to watchman lazy mode. Git projects using file_watcher=watchman and lazy_mode=fs can now upgrade to lazy_mode=watchman.
Add codemod to convert $ObjMapi<T, <K>(K) => K> to $KeyMirror<T>. Run it via flow codemod key-mirror [OPTION]... [FILE].

Library Definitions:

Add devicePixelRatio global (thanks @Brianzchen)

0.158.0

New Features:

Add support for private class methods, implementing both the instance and static proposals! (thanks to our intern, @SamChou19815)
Add an "Organize Imports" code action which sorts and groups the imports at the top of the file.
Add an "Add all missing imports" code action which auto-imports all unambiguously undefined variables in the given file.

Notable changes:

Fix a bug where private class fields like #foo: () => 'foo' could not be called, leading to workarounds like this.#foo.call(this)
Fix a crash when hovering over a type annotation that is recursive in a parameter default
Fix a crash when autocompleting inside an object literal with a recursive type
Fix order of auto-import code actions
Fix issue with utility types nested in $NonMaybeType<...>
Fix Go to Definition on module reference strings (e.g. if module.system.haste.module_ref_prefix=m#, "m#Foo" will jump to the Foo module)
The file_watcher.watchman.survive_restarts setting is now true by default. This setting is deprecated and will be removed in a future version.
The --retry-if-init CLI flag has been removed. Setting it to false previously caused the command to error if the server was already running but not yet ready. Instead, use --timeout to avoid waiting too long.

Parser:

Significantly improved parser performance
Private methods are no longer a parse error

flow-remove-types:

Fix to remove variance sigils on class properties (thanks @mischnic)

0.157.0

Likely to cause new Flow errors:

Add a new error for unreachable code occurring in a loop after a conditional with mixed break and continue branches.

New Features:

LSP extract to function/method/constant/class fields/type alias is enabled by default. These refactors will show up under after selecting some code. They can be disabled by adding experimental.refactor=false to the .flowconfig. (thanks to our intern, @SamChou19815)
Add an eslint plugin, eslint-plugin-fb-flow, for eslint rules from the Flow team. The first rule is use-indexed-access-type to encourage the use of indexed access types (https://flow.org/en/docs/types/indexed-access/).
Add another eslint rule to lint against explicit exact by default syntax (fixes #8612).
Add a command to update-suppressions to the flow tool (fixes #8384).

Library Definitions:

Better coverage of the MediaTrackSettings API.
Improve types for ResizeObserver (fixes #8693).

Misc:

Upgrade to OCaml 4.10.2.

0.156.0

New Features:

Cross-module autocompletion and code actions that fix missing import declarations are now enabled by default. This was experimental since 0.143.0. Can be disabled per project with the autoimports=false flowconfig option, or per user with the flow.suggest.autoImports LSP configuration.

Library Definitions:

Enhance the builtin Node.js module types (thanks @isker).

0.155.1

Fix bug in shared memory garbage collection which caused Flow to crash.

0.155.0

Likely to cause new Flow errors:

Ban the use of this inside of object methods (see https://medium.com/flow-type/sound-typing-for-this-in-flow-d62db2af969e).

Notable bug fixes:

Fix a bug with name collisions in declared modules (fixes #8604).
Prevent showing identifiers for autocomplete of method names.
Preserve interfaces when used with the $ReadOnly utility

Library Definitions:

Add support for fs promises (thanks @atabel).

0.154.0

Likely to cause new Flow errors:

Added type checking for this parameters of functions (see https://medium.com/flow-type/sound-typing-for-this-in-flow-d62db2af969e).

New Features:

Added codemod to cleanup the use of soon to be deprecated existential type (e.g. *). It can be run via flow codemod replace-existentials --write ..
Added IDE quickfix for optional chaining when accessing an object that is nullable.

Notable bug fixes:

Fixed crash related to a missing method unbinding error case.
Fixed crash from calling the flow binary with an unknown argument.

Library Definitions:

Fix MediaSource readyState to match the spec, "opened" -> "open" (thanks @sompylasar).

Misc:

Remove experimental.this_annot config option and permanently enable now that our core lib defs rely on this feature.
Remove previously defaulted on new_check config option for the new check mode that uses significantly less RAM for large projects.

0.153.0

Likely to cause new Flow errors:

Enforce that interfaces are supertypes of objects and classes. Also prevent objects from being a supertype of a class.
Improve soundness of this typing by banning the unbinding of class methods (see https://medium.com/flow-type/sound-typing-for-this-in-flow-d62db2af969e).

New Features:

You can require an exhaustive check of an enum in a switch statement that includes a default branch. This is enabled with the comment // flowlint-next-line require-explicit-enum-switch-cases:error.
Autocomplete for object keys.
Introduce a flow fix command to apply autofixes from the command line to a list of files.
Enable a new check mode that uses significantly less RAM for large projects.

Notable bug fixes:

Fix a bug that allowed nested anonymous functions inside a default export.
Using the --ignore-version flag also prevents Flow from crashing when an invalid option is in the .flowconfig.

Library Definitions:

Support the options parameter for require.resolve. (thanks @skeggse)
Return a TypedArray instead of void from TypedArray sort function. (thanks @goodmind)
Return a DOMStringList instead of a string array from IDBDatabase.objectStoreNames. (thanks @bnelo12)

Misc:

Add --binary to the flow version command to see the path to the current binary.
Prevent the LSP from exiting when an invalid option is in the .flowconfig.
Autofix for a method-unbinding error to replace the method with an arrow function.
Autofix for a class-object-subtyping error to replace the object type with an interface.

0.152.0

Likely to cause new Flow errors:

Fixed an issue with refinement invalidation for variables without initializers or that are written to before they're declared. You may see both new errors from improved invalidations, and some unnecessary invalidations will be removed.

Notable bug fixes:

Fixed the behavior of unary not operator on type applications

Library Definitions:

Add Symbol.asyncIterator (thanks @goodmind)
Add missing methods to HTMLTableSectionElement (thanks @vitoreiji)
Fix nullability of various DOM table properties
Fix type of MediaRecorder constructor options
Improve node crypto types (thanks @juodumas)
Improve node cluster.worker type (thanks @magicmark)

0.151.0

New Features:

Add Smart Select IDE feature. This allows you to expand or shrink your selection based on the surrounding code. For example you might expand from an identifier to the containing expression, to the whole statement. In vscode, you can do this via ctrl+shift+cmd+(left or right arrow).
Add Enum getName(value: TEnum): string method. Given a value of the enum type, this returns the string representation of the name of the member. You will also need to update to the latest flow-enums-runtime package.

Notable bug fixes:

Update annotate-exports codemod to add lowercase react instead of React (thanks @meandmax!).

Misc:

Improve developer setup on Windows by exposing internal setup scripts and updating the Windows setup README.md.
Enable cache_live_errors_artifacts and cache_signature_help_artifacts config options by default.
Improve performance and reliability of Watchman integration.
Error for use of super outside of class methods added in 0.148.0 has been removed.

Library Definitions:

Update React types for React 18 release.
Update the Map, WeakMap, Set and WeakSet constructors' iterable param to be optional as per the ECMAScript spec.
Add typing for PageTransitionEvent as per the HTML spec (thanks @bripkens!).
Added support for stepUp and stepDown in HTMLInputElement declaration as per the HTML spec.

0.150.0

New Features:

Suggest string literals in autocomplete in positions where string literal unions are expected.

Notable bug fixes:

Use bracket syntax for autocomplete when autocompleting a property name that is not a valid identifier.
Require the "main" property of package.json to have a valid Flow extension for the file to be considered.
Fall back to .js extensions even when a module name looks like a resource file (e.g. ./a.svg will look to ./a.svg.js).
Fix a crash that occurred when checking a this parameter with a generic.
Make $NonMaybeType<empty> evaluate to empty.
Honor format config options in more cases in code actions and codemods.
Fix PointerCapture library definitions (thanks @malectro!).

Misc:

Remove experimental observedBits param to React's useContext libdef.
Add caching to live errors and signature help, to speed up IDE interactions (currently behind temporary flags).
Introduce format.bracket_spacing option which affects code output (such as code actions).
Improve the sorting of autoimport suggestions that have the same name, so that the module's file extension is not considered.

0.149.0

New Features:

Added autocomplete for member expressions that use bracket notation

Notable bug fixes:

Improved typechecking for expressions like obj[Symbol.iterator]. This is now correctly typed as the object type's @@iterator property.

Misc:

Removed esproposal.class_instance_fields, esproposal.class_static_fields, esproposal.decorators, esproposal.export_star_as, esproposal.nullish_coalescing and esproposal.optional_chaining flowconfig options which have been deprecated since v0.135.

0.148.0

Likely to cause new Flow errors:

We now error if the RHS of instanceof is not an object, e.g. x instanceof null. This catches 'instanceof' is not an object runtime errors.
Using super outside of class methods, e.g. in arrow and function properties, is now an error.

Library Definitions:

Add static from to stream$Readable (thanks @agalatan).

Notable bug fixes:

Fixed autoimports to not suggest vars already in scope.
Fixed autoimports to reuse existing imports if they exist.
Fixed autoimports to correctly sort imports of the same name.
Fixed null from being a subtype of an empty interface e.g. (null: interface {}) // Error (thanks @mrtnzlml for the report).

Misc:

flow refactor experimental command removed. Instead, use the LSP textDocument/rename request.
Removed the experimental.new_signatures flowconfig option as well as the --old-signatures opt-out command line flag.

0.147.0

Likely to cause new Flow errors:

This release includes some significant architectural changes that fix bugs, improves performance, and may cause Flow to find more errors.
Fixed a bug where "maybe types" like ?{c: number} did not properly error when used with utility types $PropertyType and $ElementType. null and undefined do not have properties nor elements.
Object.prototype properties like toString and hasOwnProperty are no longer allowed to be accessed as global variables, which was previously allowed because window is an object.

New Features:

Added Linux ARM64 support. A precompiled binary is provided, including through flow-bin.

Notable bug fixes:

Improved the error message when getting properties on null and undefined
Fixed running flow lsp on symlinked root directories

Misc:

Removed support for @flow weak, which was a rarely-used mode in which missing annotations were treated as any. It is equivalent to suppressing any type errors that result from just using @flow. So to migrate, suppress the new errors and remove weak.
Fixed a bug in which the built-in library definitions could be missing
Fixed an obscure crash when using Watchman and Mercurial
Added support for a flow.suggest.autoImports LSP setting to disable autoimport suggestions in autocomplete (currently requires autoimports=true to be enabled in .flowconfig)

Library definitions:

Added support for node's assert strict mode (thanks @goodmind)
Added support for .webp resources (thanks @TomasBarry)
Added missing Brotli APIs to the zlib module (thanks @isker)
Added missing methods to tty.WriteStream (thanks @reyronald)
Changed process.exit to return empty

Parser:

Fixed a regression in 0.146 where flow_parser.js defined some global variables

0.146.0

Likely to cause new Flow errors:

The rules around when refinements are invalidated have been strengthened for increased accuracy, and Flow will now invalidate refinements in a number of locations where they were not previously invalidated. This may expose new Flow errors due to variables that were previously refined to a more specific type now being treated as the unrefined, more general type.
Object literals now have covariant subtyping with interfaces
Interface optional properties are no longer unsafely covariant in all cases
Interfaces with indexed properties are now properly supertypes of classes/objects with named properties matching the indexer
interface {} is no longer a supertype of void and mixed

Notable bug fixes:

Properties that begin with a . were sometimes being ignored by Flow, which has now been fixed

Library Definitions:

Added missing attributes to PerformanceResourceTiming library definition (thanks @MIGreenberg)
Added withFileTypes option to readdir and readdirSync (thanks @mrtnzlml)
Added composedPath function to Event library definition (thanks @Brianzchen)
Added JSDocs to Event library definition and made most properties read-only

Misc:

New flowconfig options to set garbage collector parameters for workers. The new flowconfig options are gc.worker.custom_major_ratio, gc.worker.custom_minor_ratio, gc.worker.custom_minor_max_size, gc.worker.minor_heap_size, gc.worker.major_heap_increment, gc.worker.space_overhead, and gc.worker.window_size.

0.145.0

Likely to cause new Flow errors:

Correctly typecheck destructuring defaults in function params. e.g. function i({name = 1}: {| name?: string |}) {} was previously not an error.

New Features:

New options added to flow_parser.js to support configuring comment output. comments enables comment attachment and all_comments enables the legacy list of all comments.

Notable bug fixes:

Correctly handle import paths within node_modules for auto imports.
Fixed a bug in Go to Definition on imported values that skipped all the way to the value's type definition instead of the value's definition.

0.144.1

Fixed a bug in autoimport code actions that resulted in import type { type T } ...

0.144.0

Improved generic type checking launched in v0.140. The deprecated implementation and the temporary generate_tests=true flowconfig option have now been removed.
Fixed an issue with logical operators (&&, ||, ??) and union types
Object rest properties (let {foo, ...rest} = obj) now retain the indexer of the object being destructured. In this example, if obj is {[string]: string}, then rest is also {[string]: string}.
Made the parser recover gracefully in some cases when in the middle of typing, allowing language services to work better in the rest of the file
Improved experimental cross-module autocomplete to include globals and built-in modules. Can be enabled with the autoimports=true flowconfig option.
Fixed a bug so that the server no longer exits when merely touch-ing the .flowconfig without changing it

0.143.1

Notable bug fixes:

Fixed crash on Windows and crash on Linux introduced in 0.143.0

0.143.0

Likely to cause new Flow errors:

Support for Classic mode has been dropped and Types-First mode is now always enabled (Types-First has been the default mode since v0.134). The types_first and well_formed_exports flowconfig options are no longer recognized. See https://medium.com/flow-type/types-first-a-scalable-new-architecture-for-flow-3d8c7ba1d4eb/ for more about Types-First mode.
Previously, errors in library files were sometimes being missed due to a bug. This has been fixed, which may expose errors in library files that were not previously being reported.
Import statements are no longer allowed at the toplevel of library files. To use import statements in library files they must appear within a "declare module".

New Features:

Added experimental support for cross-module autocompletion and code actions that fix missing import declarations. Can be enabled with the autoimports=true flowconfig option.
Added --sharedmem-heap-size CLI flag and FLOW_SHAREDMEM_HEAP_SIZE environment variable, which can be used instead of the sharedmem.heap_size flowconfig option for setting the amount of shared memory available.

Misc:

Added ErrorEvent library definition (thanks @kegluneq)

0.142.0

Likely to cause new Flow errors:

Disallow flowing functions or inexact objects to indexed objects to improve object soundness. This can cause errors if you are passing a function or inexact objects when an indexed object is expected.
Flow now processes imports before checking the body of a file. In some rare cases this can expose previously skipped errors due to the processing order.

Notable bug fixes:

Fix No available version of ocaml-base-compiler satisfies the constraints error from make all-homebrew (thanks @bayandin).

0.141.0

Improved inference of chained generic method calls, such as Array methods. For example, given [1, 2].map(a => a).forEach(b => b), Flow now infers that b is a number rather than any | number.
Fixed non-termination bugs involving recursive types
Fixed a non-termination bug involving implicit instantiation with _
Fixed autocomplete so it no longer inserts a = in JSX attributes that already have one
Hovering over a use of an opaque type now includes the type's documentation

0.140.0

Likely to cause new Flow errors:

New generic type checking is now enabled by default, and has to be explicitly disabled with generate_tests=true in a flowconfig if desired. See https://medium.com/flow-type/flows-improved-handling-of-generic-types-b5909cc5e3c5 for more about Flow's new handling of generic types.

Notable bug fixes:

Fixed "Could not locate flowlib files" errors when multiple users run Flow on the same machine
Fixed autocomplete and hover for imported enum types
Fixed autocomplete suggesting types in value positions
Fixed a bug where correct non-boolean predicate functions were rejected

Parser:

ESTree AST now uses Literal node for init of boolean enum members

Misc:

Updated ShadowRoot library definition (thanks @Brianzchen)

0.139.0

New Features:

Support for this annotations in functions, like function f(this: {foo: string}, param1: string): string { return this.foo; }
The experimental.abstract_locations config option is now true by default, as it enables significant performance improvements. This option is now deprecated and will be removed in a coming version.

Notable bug fixes:

Fixed a false positive when a bounded generic like K: string flows into $Keys<{[K]: ...}>
Fixed a false positive when a bounded generic like K: 'literal' is checked against itself like k === 'literal'
Fixed autocomplete inside of JSX attribute values
Fixed autocomplete of properties of interfaces

Misc:

Updated flow-remove-types to support this parameters
Added SpeechRecognition definitions (thanks @ayshiff)

0.138.0

Likely to cause new Flow errors:

Improved positioning of existing errors involving unions or intersections. Will not cause new errors, but may require suppressions to be moved.

Bug Fixes:

Fixed soundness bug involving union and intersection types
Improved performance of typechecking exact objects against $Exact

Editor Integration:

Fixed hover and go-to-definition over the LSP when the cursor is at the end of a token
Fixed type coverage over the LSP when using all=true (thanks @mochja)
Fixed an issue with running the flow CLI at the same time as VS Code caused by case-insensitivity of Windows paths

Library Definitions:

Added inputType to InputEvent (thanks @Brianzchen)
Added intersectsNode to Range (thanks @Brianzchen)

Misc:

Built-in library definitions are now extracted to a consistent temp directory, like /tmp/flow/flowlibs_<HASH>, to take up less space.

0.137.0

New Features:

Show code actions in the IDE for some parse errors with suggested fixes.

Notable bug fixes:

Fixed infinite recursion case involving object rest destructuring.

Library Definitions:

Changed return type of Buffer.write to number (thanks @fedotov).

Misc:

Saved state files generated on one platform can now be used on another platform using the same version of Flow.
Improved saved state memory use and startup time with more efficient handling of file paths.
Improved error message when attempting to bind a class type.
Minimum supported MacOS version is now 10.13.

0.136.0

Likely to cause new Flow errors:

Flow now raises errors when generic type variables escape out of the scope in which they were defined.

Notable bug fixes:

Fix a race condition related to saved state and cancelable rechecks that caused internal errors of Requires_not_found or Sig_requires_not_found.

Misc:

Added documentation to core.js builtins.
Add optional propTypes to React.AbstractComponentStatics (thanks @brianzchen).

0.135.0

Likely to cause new Flow errors:

Turned the untyped-type-import lint into an error by default. In a later release, we will turn this into a regular type error instead of a lint error.

Misc:

Improved exhaustiveness checking in switch statements.
Improved autocomplete results to show documentation in more cases.
Removed esproposal configuration options from the .flowconfig format.
Added library definition for MediaRecorder API.
Fixed Object.freeze to no longer incorrectly convert an inexact object into an exact object.
Fixed handling of exported objects with spreads in types-first mode.

0.134.0

Likely to cause new Flow errors:

Types-first is now the default mode, and has to be explicitly disabled with types_first=false in a flowconfig if desired. See https://medium.com/flow-type/types-first-a-scalable-new-architecture-for-flow-3d8c7ba1d4eb for more about types-first.

New Features:

Added a file_watcher.watchman.mergebase_with flowconfig option and --file-watcher-mergebase-with CLI option to choose a tracking bookmark for Watchman to compute the mergebase against.
Show JSDoc documentation in type declarations.

Misc:

Improved the consistency of what operation is pointed to in error messages, to ensure they aren't dependent on the order in which constraints are generated.
Filtered "did you mean" code actions to only send actions on code under cursor.
Changed the return type of fileURLToPath (thanks @aaronasachimp)

0.133.0

Likely to cause new Flow errors:

Improve reliability of inclusion check in strict equality conditionals.

New Features:

Show JSDoc documentation in hover, autocomplete, and signature help.

Notable bug fixes:

Fix a bug related to saved state that could cause crashes during initialization.
Fix a bug related to saved state that could cause internal errors during rechecks.
Fix crash with flow focus-check when a focused file has a syntax error.

Misc:

Remove the null-void-addition lint now that it has been subsumed by an ordinary type error.
Improved signature help for inlined tuple rest params.
Enable the following language features by default, since they have reached stage 4:
- export *
- Nullish coalescing
- Optional chaining
Remove deprecated, nonstandard itemType and inlineDetail fields from the LSP completionItem implementation.
Exit the worker processes gracefully when the main process crashes.

0.132.0

Likely to cause new Flow errors:

Disallow using any-typed values type annotations. example

New Features:

Added warnings against suppressions without error codes

Notable bug fixes:

Fixed some crashes when Flow could not connect to Watchman

Misc:

Fix syntax errors in README badges (thanks @jamesisaac)

0.131.0

"Go to Definition" on a JSX attribute name now jumps to the prop's type.
Improved error messages for non-strict equality checks
Fixed a bug that prevented the use of module in ES modules when well_formed_exports is enabled.
Fixed an issue that allowed types-first mode to be enabled (types_first=true) when using well_formed_exports.include. For types-first to function properly, the entire project's exports must be well-formed, not just specific paths.
Fixed a similar issue so that --types-first on the command line overrides well_formed_exports=false in .flowconfig. This makes it easier to test types-first mode.
The unsafe-addition lint now defaults to being an error. It will be converted from a lint to a type error in the future.
Removed dynamic-export lint which does not work in types-first mode
Improved initialization performance

0.130.0

New Features:

Improved IDE get definition behavior of imported names by jumping past module.exports = ... to the actual definition.

Library Definitions:

Made offset an optional argument for the sized Buffer read/writes (thanks @isker!)

Misc:

Removed deprecated blacklist/whitelist aliases for config options, use includes/excludes instead (see updated docs: for Types First and flowconfig options).

0.129.0

Likely to cause new Flow errors:

$Keys<...> will no longer include non-own instance properties, matching the behavior of Object.keys.

Notable bug fixes:

Fixed an incremental rechecking issue caused by the hashes of type destructors

Misc:

Updated the type of cast and isValid to take in optional representation type for enums
Added missing links to Medium articles from flow.org (thanks @jamesisaac)

0.128.0

New Features:

Enable LSP support for autofix exports by default.
Added a lint rule to ensure exports named default were exported with export default syntax.
Enabled support for JSDoc in some LSP results.

Library Definitions:

Add parameter to MediaStreamTrack.applyConstraints()

Parser:

Fix parsing async arrow functions with multiple type parameters

Config:

Remove types-first flag aliases prefixed with "experimental"
Replace well_formed_exports.whitelist with well_formed_exports.includes

0.127.0

Misc:

Improvements to editor in flow.org/try, including syntax highlighting
Fix broken links in docs
Standardized error suppression syntax, added ability to suppress errors based on error codes

Library Definitions:

Add KD functions
Add onended callback to OscillatorNode
Make 3rd parameter of node.js symlinkSync optional

0.126.1

Fixed an issue where changing .flowconfig or package.json and then running flow status would get stuck in a restart loop, if using lazy mode with Watchman and Mercurial.

0.126.0

Allow indexers in exact object annotations
Fixed type-at-pos in flow.org/try
Improved language of LSP window/showStatus responses
Improved control flow handing in logical expressions
Allow applying utility types to opaque types in the defining module
Added HeadersInit and signal to Fetch API (thanks @andretshurotshka!)
Added replace method to DOMTokenList (thanks @w01fgang!)

0.125.1

Publishing of flow-bin v0.125.0 failed to include the binaries. Oops!

0.125.0

Likely to cause new Flow errors:

Fixed signatures generated in types-first for classes with non-trivial expressions in their extends clauses, which were before treated like extending any.

New features:

Types-first is no longer an experimental mode, but is now fully supported!

Notable bug fixes:

Fixed a soundness bug that allowed refined property expressions to be treated as empty in some circumstances.

Misc:

Added $FlowIssue and $FlowExpectedError as default suppression comments.
Improved quality of AST and comment printing and layout.
Improved quality of type printing for type-at-pos and friends.
Add '.cjs' as a default extension.

Parser:

Fixed ranges of sequence and assignment expressions that contain grouping parentheses.
Disallowed newline after type keyword in type aliases.

Library definitions:

Added headers timeout to http & https server (thanks, @mattconde!)
Added flushHeaders to http$ServerResponse (thanks, @gajus!)
Changed thisArg parameter for array callback functions (e.g. map, reduce) to mixed.
Added more precise typing for common cases of Array.flat.

0.124.0

Library Definitions:

Add StorageEvent overloads.

Misc:

Use textEdit in identifier completion item responses over the LSP, to avoid possible behavior differences with different LSP clients.
Various improvements to the pretty-printer.
A small improvement in completeness when handling the "not" operation in the presence of unions of literal types.
Wrap the usage message to 100 chars.
Fix a poorly-positioned error related to polymorphic function types.

Parser:

Improved some parser error messages by treating the end of file as a valid end of object property.
Updated ESTree output for import expressions to match the spec.

flow-dev-tools:

Add the update-suppressions subcommand.

0.123.0

New Features:

Added support for the babel react jsx transform with the react.runtime flowconfig option. When set to automatic the transform auto-imports jsx (the replacement for createElement) under a fresh name (unique of all existing names in the source). When set to classic it will continue transpiling jsx to React.createElement.

Notable bug fixes:

We are now treating $Exact<T|U> as $Exact<T>|$Exact<U>. Before, when we checked against this type we would treat it like $Exact<T> & $Exact<U>, instead.
Fix to prevent infinitely expanding recursive type applications of arrays (example).

Misc:

Improvements in comment attachment in various kinds of AST nodes. These will help improve the accuracy of error suppression and various services that inspect comments (printing, showing documentation, etc.).
$Exports<'m'> will now lookup module 'm' directly in builtins, instead of the environment.
Extend union optimizations used on maybe predicate to also be applied in exists predicates.

Parser:

The range around type cast expression now includes the parentheses. This follows the pattern of other constructs with mandatory punctuation like blocks, objects or arrays.

0.122.0

Likely to cause new Flow errors:

Moved react-dom modules out of built-in libdefs. They are now available on flow-typed.

To install the libdefs, run flow-typed install --flow-version=0.122.0 react-dom@16.13.0

This diff is necessary to allow React to make breaking changes to react-dom without force-pinning Flow users to the latest version of react-dom.
Fixed an unsoundness in union and intersection spreads
Improved object spread-related error messages, which may cause suppressed errors to become unsuppressed because they moved to better locations

Improved Editor Integration:

Removed completion of function param snippets. Before, f<tab> might complete foo(aParam), which is problematic if you don't want to call the function. Now it just completes foo. These snippets are further obviated by signature help, which appears when you type (.
Improved accuracy of "Did You Mean?" Quick Fixes for object property type errors
Added support for Go to Definition on export-from declarations (export { HERE } from ...)
Fixed tracking of open files in LSP clients which could cause stale errors in the IDE

Misc:

Added HTMLUnknownElement and support for custom elements to document.createElement (thanks @YevhenKap!)
Fixed --saved-state-no-fallback flag so that Flow exits when saved state is not found
Fixed an issue where an invalid package.json would prevent generating saved state
Fixed a potential deadlock when communicating with Watchman
Fixed types-first signatures for CJS requires introduced with var
Enabling experimental.types_first now implies experimental.well_formed_exports

Parser:

Renamed RestProperty to RestElement to match estree
Changed 'for await' nodes from ForAwaitStatement to ForOf with await: true to match estree

0.121.0

Highlights:

Made several improvements to errors:
- Restricted errors to only be suppressible at the error's primary location (see blog post)
- Fixed the error grouping logic to no longer group unrelated errors that happen to share a location
- Changed the order of locations printed in error messages so that the primary location is always printed first (which is the one you need to suppress if you want to do that)

Misc:

Fixed the add-comments script to add comments on JSXText
Fixed a crash when TMPDIR exceeded 83 characters
Stopped parsing .flow files without @flow

Lib defs:

Improved definitions for Node's url.parse function (thanks @chicoxyzzy)
Added missing console methods (thanks @goodmind)
Added Element.hasAttributes()
Added string index to NamedNodeMap
Fixed Element.querySelector() and Element.querySelectorAll() overloads
Added type definition for Promise.allSettled()

0.120.1

Likely to cause new Flow errors:

$Shape types now carry more information about the locations of errors that involve them, which may invalidate old suppressions.
any-typed values are now refined by primitive typeof checks (example).

New Features:

"Did You Mean?" IDE feature that suggests corrections to your code as you type. For example, if you write foo.bar on an object foo that doesn’t have a field named bar but does have a field named baz, the quick fix will apply that suggestion.
declare can now be used on class fields. When Flow implemented the class fields proposal, uninitialized class fields (e.g. class C { foo }) were not allowed, so we used that syntax for type-only declarations (class C { foo: string }). Since then, they now are initialized to undefined and Babel 8 will start to leave them. To maintain existing behavior, change uninitialized fields to use the declare keyword, signalling to Babel that they should be stripped (class C { declare foo: string }).

Notable bug fixes:

Fixed possible stack overflow from overly long traces.
Fixed infinite recursion case involving polymorphic types.
Various improvements to the output of type-at-pos and signatureHelp

Misc:

Fixed libdefs of Notification to have read-only fields

0.119.1

Notable bug fixes:

Fixed a bug that would cause Flow to crash on some rechecks.

0.119.0

New Features:

Implemented LSP textDocument/signatureHelp, which shows parameter hints when you are within an argument list in a call or new expression.

Misc:

Removed the minimal_merge flag
Added optional callback for node dgram socket.close function (thanks @davidnaas!)

Parser:

Made arguments on new expressions optional

0.118.0

New Features:

Autocomplete for nullable objects will now suggest completions with optional chaining syntax
Added a new lint, unsafe-addition, which warns if either operand of an addition is null or void

Breaking change:

The output of flow autocomplete --json no longer includes location information. This might affect some IDE integrations which have not yet updated to Flow's LSP server. The LSP integration is unaffected.

Misc:

Added the dir field to the Document libdef (Thanks, @lukeapage!)

0.117.1

Improved timeouts related to Watchman that could cause the server to fail to start if Watchman is slow to respond. Added a file_watcher_timeout .flowconfig option and --file-watcher-timeout argument to flow start and flow server, which defaults to 120 seconds.

0.117.0

Notable bug fixes:

Improved the behavior of $Diff and other type destructors when applied to unions.

Misc:

Improved error messages when attempting to use watchman without watchman installed.
Removed uses of Symbol from libdefs in favor of symbol.
Fixed definition of fs.promises.mkdir (thanks @gabrielrumiranda!)

0.116.1

Notable bug fixes:

Fix #8259.

0.116.0

New Features:

Flow now suggests similar names (if applicable) when issuing missing property errors.

Misc:

Added a rudimentary libdef for Array.prototype.flat (#8237, thanks @nnmrts!).
Fixed a bug that in certain specific cases led to an expression getting typed as empty rather than any.

Parser:

> and } in JSX child text is a parse error.

0.115.0

Likely to cause new Flow errors:

In Types First mode, exported classes and functions cannot be reassigned.
Added similar checking of symbol in non-strict equality as other primitive types (example). We allow for non-strict equality checks between two symbol types, and between a symbol and null/void (example).
We now write properties to unsealed objects recorded during speculative checking, after speculation has succeeded.
Added more complete handling for indexers in spreads:
- Use the indexer on the right, when the left is empty, exact and has no indexer. [example]
- Allow matching indexers to be spread. [example]
Removed the non-array-spread lint, and replaced it with a new .flowconfig option, babel_loose_array_spread.

Notable bug fixes:

Improved union optimization for non-enum union equality checks.
Fixed a bug in which $Exact<T> failed when T resolved to a Shape<T>. [example]

Misc:

Improved performance of calculating the set of files that need to be checked.
Cached the typed AST as part of the persistent connection client info, to speed up requests coming from an LSP client (DefinitionRequest, Hover, TypeCoverage).
Improved dependency graph construction times, by using Hashtbl and mutable back edges.
Improved tool add-comments by
- using flowlint-next-line instead of adding full $FlowFixMe suppressions on lints, and
- making the rule for inserting comments inside JSX elements also apply to JSX fragments.
Upgraded to sedlex 2.1.

0.114.1

Upgrade internal dependencies sedlex and lwt. Lwt includes a fix to stack traces that we need to debug crash logs.

0.114.0

Likely to cause new Flow errors:

Fixed the order of evaluation of JSX attributes and children to match runtime, so that refinements work properly. This can lead to new errors if refined values are used as children, and a function is called within an attribute, because the refinements are now invalidated in a different order. On the other hand, calling a function in a child no longer incorrectly invalidates values used as attributes. [example]
Added support for refinements on optional chains! Now, examples like if (x?.y) and if (x?.y === 42) understand that x is truthy in the consequent. Flow also now refines subexpressions of optional chains. For example, given obj?.fun(obj.value), the second obj is known to be truthy since it is only reachable if the earlier obj?.fun is also truthy; therefore, you don't need to optional-chain it again (obj?.value).

New Features:

Added typechecking support for symbol! Note that just like number is not a Number, and vice versa, symbol is not a Symbol. In the core lib defs, we change usage of Symbol to symbol & Symbol. Eventually, it will just be symbol. The symbol & Symbol is temporary for one version to allow for existing usage of the : Symbol to be codemodded to : symbol.

Bug fixes:

Optimized typechecking pointwise-equal unions, from O(n^2) to O(n)
Optimized reverse dependency computation, greatly reducing the time spent computing what needs to be rechecked after an edit
Fixed an error message that pointed at the definition of a write-only property instead of the callsite that tried to read it [example]
Fixed an issue where coverage results could be incorrect in Types First mode when a file is rechecked
Fixed an issue when Flow prints evaluated types like $ElementType, that led to printing empty instead. This improves hover tooltips.
Fixed several issues causing various processes to die uncleanly while exiting.

0.113.0

Bug fixes:

Fixed autocomplete when at the end of file
Stopped filtering lints (e.g. signature-verification-failure) out when running flow check-contents

Parser:

Added support for export default class implements Foo {} [example]
Added support for type parameters in anonymous class expressions [example] (thanks @nicolo-ribaudo)

Library definitions:

Fixed type of navigator.mediaDevices.getUserMedia (thanks @thecotne)
Added missing InputEvent to removeEventListener
Updated type of ServiceWorkerContainer.getRegistration

Misc:

Added module.system.node.main_field option (see #8128 for more details)

0.112.0

Likely to cause new Flow errors:

JSX expressions now use the new spread semantics added in v0.111.0
Attempting to write to or update a read-only property using operator assignment is now a type error

New Features:

New ambiguous-object-type lint warning against use of {} object types (prefer {||} or {...} instead, even when exact-by-default is enabled).
Improved support for optional chaining

Notable bug fixes:

Malformed type annotations no longer trigger the unclear-type lint error
Builtin classes can no longer be extended

Misc:

Improved performance for utility types and refinements in unions
Various improvements to autocomplete and get-def IDE services
Add useDeferredValue and useTransition to React library definitions
Add bytesWritten function to library definition (thanks @farzonl)

0.111.3

Notable bug fixes:

0.111.2 did not include the commit that bumped the Flow version inside of Flow. (@jbrown215 was a clown)

0.111.2

Notable bug fixes:

Spread performance improvements that can prevent timeouts in extreme cases.

0.111.1

New features:

Add module.system.node.root_relative_dirname to allow you to configure where root relative paths resolve to

Notable bug fixes:

Fixed the regex generator that handles the magic string in Flow suppression comments
Allow spreads of bools, strings, and numbers to support common React Native patterns

0.111.0

Likely to cause new Flow errors:

Fixes to object spread. See https://medium.com/flow-type/spreads-common-errors-fixes-9701012e9d58

New features:

Introduced an experimental flag (experimental.minimal_merge) that speeds up rechecks when the experimental types-first mode is in use. This flag will be turned on by default and then removed in future releases.
Turned on experimental.allow_skip_direct_dependents by default. The flag will be removed next release. This flag speeds up rechecks when the experimental types-first mode is in use.

Notable bug fixes:

Fixed a bug where the ocaml representation of union types caused crashes in rare scenarios
Fixed a bug in node module resolution which allowed module.system.node.resolve_dirname=.. For those who relied on this bug to import modules using root-relative paths, you can now use module.system.node.allow_root_relative=true. See #8156 for more details.

Misc:

Add getElementById to DocumentFragment
Add missing methods to Blob (thanks @lyleunderwood!)
Allow clients of flow status, flow check, etc. to choose character offset style
Support length refinement on tuples (thanks @ilya-bobyr)

Parser:

Allow => in objects in return types of arrow functions

0.110.1

Notable bug fixes:

No longer show live Flow errors for files without @flow, unless all=true in the .flowconfig
No longer show live Flow errors for ignored files
Re-generate live errors after a recheck

0.110.0

New Features:

Flow will now send type errors as you type to LSP clients. To disable this behavior, add experimental.disable_live_non_parse_errors=true to the [options] section of your .flowconfig.

Notable bug fixes:

Fixed file descriptor leak leading to LSP connection refusal in some cases
Improved the behavior of the [declarations] configuration in the presence of dependency cycles (thanks @STRML!)

Misc:

Added --evaluate-type-destructors to type-at-pos command.
Added --evaluate-type-destructors and --expand-type-aliases to dump-types command (thanks @goodmind!)
Changed process.env values from ?string to string|void (thanks @FireyFly!)
Improved detection of rebases when using watchman file watcher
Improved positions for error messages involving the deprecated * type

0.109.0

Likely to cause new Flow errors:

Flow was previously not typechecking delete, but now does
Object.defineProperty and similar methods now adhere more closely to the spec
Allow defaults for properties that may not exist in React components

Notable bug fixes:

$NonMaybeType<mixed> no longer includes null and undefined (Thanks @goodmind)

Misc:

Deprecated $Supertype and $Subtype utilities are now removed entirely
flow-upgrade should now use https over git (Thanks @lukeapage)
Autocomplete now fires on spaces in JSX
Better error messages when a value is used as a type

Performance:

Reduced memory usage by using a more compact representation for code locations
Types-first should no longer check direct dependents of files where the signature has not changed

Library Definitions

Add a number of CSSOM interfaces + fix HTMLStyleElement.sheet type (Thanks @kof)
Add AsyncIterator to node.js (Thanks @goodmind)
Add undefined to prelude (Thanks @goodmind)
Add String.prototype.matchAll (Thanks @goodmind)

0.108.0

Notable bug fixes:

Batch coverage info now persists through rechecks.
When a file with @preventMunge is in a cycle with a file that does not, the un-munged file no longer has exported munged properties checked for annotations.
Fixed a bug where autocomplete would not be triggered in a file with Windows-style line endings.
Improved error messages for uses of $ObjMap, $ObjMapi, $TupleMap, and $Call with incorrect arity.

Misc:

The deprecated flow ide command and associated machinery have been removed.

Library definitions:

Added the missing parts of the Pointer Lock spec to libdefs.
Thanks to @lyleunderwood for adding scrolling support to libdefs.
Thanks to @goodmind for adding a definition for undefined to the prelude.

0.107.0

New Features:

Implement type refinements for property accesses through brackets (#7597, thanks @goodmind).

Notable bug fixes:

Fix several issues with autocomplete that prevented optional properties, type aliases to utility types, types with default type arguments, and several other edge cases from being autocompleted.
Fix Not_expects_bounds crash with %checks (#7863).

Misc:

Make the LSP textDocument/documentHighlight request serviceable while Flow is in the middle of a recheck.
Fix minor off-by-one error which caused some parts of traces to be pruned when using the --traces flag.
A minor improvement in error messages when undefined is involved.
Reduce memory usage slightly when abstract locations are enabled.
Prevent log spew when flow lsp is started while the server is initializing.
Improve completeness of sighashing. We recently observed an incremental bug caused by incomplete sighashing. This speculatively addresses similar potentially problematic cases.

Library Definitions:

Add webkitGetAsEntry to DataTransferItem.
Switch several properties to optional in Notification and NotificationOptions (#8032, thanks @pauldijou).
Map/Set fix symbols (#7560, thanks @goodmind).
Fix Array#reduce and Array#reduceRight (#7902, thanks @goodmind).

0.106.3

We found and fixed a bug introduced in 0.105.0. Some internal code was using a hashing function and assumed collisions were far less likely than they proved to be. This could lead to random nonsensical errors which would then disappear, usually involving missing object properties. This likely only affected extremely large projects.

0.106.2

Fixed the stack overflow reported by #8037. Thanks @lukeapage for isolating the repro!

0.106.1

Forgot to cherry-pick [rollouts] (an experimental new .flowconfig section) into v0.106.0

0.106.0

Likely to cause new Flow errors:

We're starting to make changes to how Flow models object spreads. For more see this announcement
Updated parsing of the experimental nullish coalescing ?? operator. It now has a lower precedence than || and &&, and parentheses are required to nest it with them.
Flow wasn't typechecking the properties of certain obscure JSX usage (namespaced identifiers, member expression with @jsx / @csx), so would miss type errors in their expressions (e.g. <a:b prop={"hello" * 10} /> now errors)

Notable bug fixes:

Fixed a bug where merge or check jobs would crash when a parse error was added to a file in a cycle. The crash was silent but unintended.
Types-first no longer ignores the munge_underscores flowconfig option

Misc:

Various libdef updates. Thanks for all the PRs!

Parser:

Improved error messages for missing semicolon
Comments are now correctly attached to break statements and array patterns
libflowparser now supports esproposal_nullish_coalescing as an option

0.105.2

v0.105.0 started running the Flow server in a cgroup on Linux distros that support cgroup v2. However, some versions of systemd, which manages cgroups, contain a bug that caused the Flow server to fail to start. This release avoids using cgroup on these systems. (#8012)

0.105.1

This was an npm-only release to fix a packaging issue. No updated binaries were published.

0.105.0

Likely to cause new Flow errors:

Types for FileReader properties and methods are now more precise (e.g., some parameters typed any are now typed ProgressEvent; some properties now have null added to their types). Thanks, @nwoltman!
The value type parameter V of $ReadOnlyMap and $ReadOnlyWeakMap is now covariant. Thanks, @goodmind!
Types for the vm module in node.js are now more precise. Thanks, @goodmind!
The deprecated $Enum<...> utility type has now been deleted. Use $Keys<...> instead.
Indexing tuples with floats is no longer allowed.

New Features:

Added support for React.Profiler (React v16.9+). Thanks, @bvaughn!
Added a --types flag to flow graph dep-graph to output only "type" dependencies: the subset of imports that the types of a module's exports depends on. (Without the flag, we output "code" dependencies: the set of all imports of a module.)
Preliminary support for automatically inserting annotations on a module's exports through LSP. Thanks to @akuhlens (summer intern with the Flow team)!
Preliminary support for definite assignment checking of class instance properties. Thanks to @pzp1997 (summer intern with the Flow team)!
Added an option to .flowconfig for exact-by-default objects.

Perf fixes:

Fixed a non-termination issue with a recursive use of mapped types.
Fixed an exponential-blowup issue with a combined use of spreads and unions.
Fixed an exponential-blowup issue with recursive use of array spreads.

Misc:

Fixed LSP init to say codeLens is not supported.
Fixed lots of cases of bad error positioning, unblocking improvements to error suppressions and error streaming. Thanks to @mvcccccc (summer intern with the Flow team)!

Parser:

Improved a bunch of "unexpected" parse errors, providing what was expected in the error message.
Fixed a bug in parsing of params in function types.

0.104.0

Likely to cause new Flow errors:

Fixed the definition of Function.prototype.apply to only accept array-like objects, not any iterable.
Improved error positioning, which may cause previously-suppressed errors to become unsuppressed.

New Features:

non-array-spread lint rule: Fires when a non-array iterable is spread. This is useful for modeling the loose: true mode of @babel/plugin-transform-spread, where such code causes a runtime error.

Notable Bug Fixes:

Fixed a performance regression when computing dependencies in large projects
Fixed built-in library definitions that needed to be explicitly inexact to pass the implicit-inexact-object linter.
Improved libdefs for many browser APIs (e.g. Media Streams, MIDI, Permissions, Workers) (#7737, #7805, #7806, thanks @goodmind!)

Misc:

Various improvements to the types-first signature generator
Improved the name of the server master process in ps
Improved the output of flow check --profile

flow-remove-types:

Remove opaque types and declare export

Parser:

Fixed a bug allowing await to be a parameter in async functions
Several improvements to the experimental comment attachment algorithm

0.103.0

New Features:

Added --types flag to flow cycle, when given only type dependencies are used to compute cycles

Notable bug fixes:

Fixed a bug when destructuring unions gave spurious errors [example]
Updated for-in/for-of head expressions to be evaluated in the correct scope [example]

Performance:

Improved the calculation for what are dependents of a file, reducing work during rechecks

Library definitions:

Added support for Array#flatMap (thanks @goodmind)
Replaced usages of Object and Function (which are aliases for any) in library definitions with any
Removed some usages of any from library definitions
Updated Function#apply to accept strictly two arguments (thanks @goodmind)
Added SpeechSynthesis definitions (thanks @goodmind)

Misc:

Updated/added HTML spec URLs in comments (thanks @kevinSuttle)
Fixed parsing of anonymous class implements clause (thanks @goodmind)
Added support for printing mixins and implements (thanks @goodmind)

0.102.0

Likely to cause new Flow errors:

Function components with no arguments get a sealed empty object type as props.
Moved MixedElement export into the module declaration, so it will now need to be qualified as React.MixedElement.

Notable bug fixes:

Fixed error positioning around utility types (e.g. $ObjMap).
Omit reporting error stack traces to end users over LSP.
Fixed bug where Flow would crash when variable has same name as a type (fixes #7825)

Misc:

Refactored coverage computation to use the typed AST. This enables coverage results over more locations that earlier.
Improved server and monitor error logging.
In typing object types as react components, account for the defaultProps property and make them compatible with React.AbstractComponent.
Optimized the way module exports are populated to prevent recursion limiter exceptions.
Improved error messages for invalid BigInts. (thanks, @goodmind!)
Hovering over an imported type alias returns its definition. (thanks, @vicapow!)
Fixed semver comparison to allow for suffixes such as rc.

Libdefs:

Remove Object type (equivalent to any) from WeakSet and $ReadOnlyWeakSet. (thanks, @goodmind!)
Add methods to Node HTTP ServerResponse type definition. (thanks, @chrislloyd!)
Add definitions for the Web Animations API. (thanks, @goodmind!)

0.101.1

Notable bug fixes:

Fixed a bug with suppressions in the experimental types-first mode.

0.101.0

Likely to cause new Flow errors:

$Keys now produces a more precise type, which may find errors where incompatible strings were passed to something expecting the $Keys of some object.

New Features:

We released a new implicit-inexact-object lint to detect when an inexact object is used without explicitly adding ... to the end of the props list. See here for context.
Function type parameters may now use default arguments. This is not yet supported by babel.

Notable bug fixes:

Fixed a bug with ranges returned by autocomplete
Fixed a bug where errors with bad locations reported over the LSP could cause the editor to clear all errors.

Misc:

React.memo and React.lazy now both allow you to specify an instance type via React.AbstractComponent.
Various performance improvements to union types.
Various libdef fixes and improvements.
Various improvements to error positioning.
The recursion limit is now configurable in the .flowconfig via recursion_limit. Most projects will not need to override this value.

Parser:

Forbid private fields named #constructor
Fix duplicate private class field validation for getters/setters
Fix parsing of private getters and setters
Function type parameters may now use default arguments.

0.100.0

Likely to cause new Flow errors:

The React$ElementType annotation, which was previously unsafe, is now strict. Before you could create an element given a component with this type using arbitrary props. To annotate any component that accepts some given props, use React$ComponentType instead. Try Flow example
The React$ComponentType annotation is now strict when used with refs. Before, it was possible to pass a ref having any type when creating an element from a component using this type. If you need to describe components that accept refs, use the React$AbstractComponent type instead. Try Flow example
The $Enum built-in type annotation is now deprecated. Please use the semantically equivalent $Keys type instead.
Destructuring patterns could previously include missing properties if the resulting binding was unused. This is now an error even when unused. Try Flow example

New Features:

You can now use the built-in type React$MixedElement as the sound superclass of all React elements. This is a type alias for React$Element<React$ElementType>.

Misc:

Add decode method to HTMLImageElement (thanks, @vicapow!)

Parser:

Handle NonOctalDecimalIntegerLiteral
Remove U+180e (Mongolian vowel separator) from list of valid whitespace code points
Remove support for legacy octal literals with numeric separators
Remove support for legacy octal bigints
Fix various issues related to automatic semicolon insertion (ASI) for class properties

0.99.1

Notable bug fixes:

Fix bug where well-formed-exports errors were reported for unchecked files

0.99.0

Likely to cause new Flow errors:

The statics of function types used to be any but are now typed as an empty object.
Recursive calls of named function expressions were previously unchecked, but are now checked.
$call property syntax, deprecated in Flow v0.75, has finally been removed.

Notable bug fixes:

Fix an issue where Flow would not catch certain errors involving React function components with unannotated props.
Fix React synthetic mouse events for drag, wheel, pointer events to give the specific native event type. (Thanks, @Kiwka!)

Misc:

Improved performance of starting a server from a saved state.

Parser:

Fix parsing of function types inside tuples inside arrow function return types.

0.98.1

Notable bug fixes:

Do not report bad module uses in unchecked files

0.98.0

Likely to cause new Flow errors:

Infer void before typechecking starts for functions without a return statement, lessening the impact of a union typechecking bug (#7322).
Fix a bug which prevented Flow from asking for required type annotations.
Turn the deprecated-utility lint on by default.
Two related changes to type refinements to fix unsoundness:
- mixed refined to an array produces a read-only array.
- mixed refined to an object produces a read-only object.

New Features:

Add the ability to exclude paths included by a previous pattern in a .flowconfig (#7317).

Notable bug fixes:

Fix a bug that led IDEs to report all code as uncovered (#7654).
Fix the untyped-import lint rule so that export [type] * from triggers it.
Flow now recognizes refinements against negative number literals.

Misc:

Exclude deprecated-utility and dynamic-export lints when applying all=setting rules (#7473).
Improve client/server version mismatch behavior so that the newest of the two is preserved, rather than the client version.
Preserve exactness of the input type when using $ObjMap or $ObjMapi (#7642).
Minor changes to metadata in the results of flow type-at-pos --json.
Batch DidOpen notifications from the IDE in order to make checking in IDE lazy mode more efficient.
When flow lsp automatically starts a server, it prefers the lazy mode set in a .flowconfig to the lazy mode passed on the CLI.
Allow lints to be explicitly set to their defaults (normally redundant lint settings are disallowed).
Fix spurious missing annotation errors when the this type is used incorrectly.
Fix a bug that made React.Element behave differently than React$Element.
Fix an edge case where object property assignments were typechecked incorrectly (#7618).
Fix an unsoundness with addition or logical operators when combined with generics (#6671, #7070).
Fix an issue which allowed read-only arrays to be written to if the index was of type any.
Fix a bug which stymied typechecking after try/catch blocks (#7530).

Libdefs:

Add document.elementsFromPoint() (#7540).
Add ConstantSourceNode (#7543).
Remove React.Suspense maxDuration attribute (#7613).

0.97.0

Likely to cause new Flow errors:

Refining a variable of type mixed with instanceof A produces type A instead empty which was produced before.
Types imported in a declare module are no longer automatically exported from that module as well.

New Features:

#7518 Adds support for LSP function parameter completion (thanks @vicapow)

Notable bug fixes:

Return a better error message when flow coverage is passed an invalid input path.
Fixed a bug in which Flow crashed on very long directory paths.
Fixed type-at-pos results when reporting the type of a callable object.

Misc:

Improvements in AST utilities: The differ got improved support in several kinds of type annotations (literals, generic identifiers, typeof, tuples and interface types). The mapper got support for qualified identifiers.
Introduces a resizable array data structure that is used in union-find.
Improved error messages around callable and indexer.
Type-at-pos now shows results without evaluating type destructors like object spread, $Diff, etc. This should lead to more compact results.
Various refactorings in the internal type language and environment.

Library definition improvements:

Updates in Node definitions. The stream definitions were updated to the latest version, and the readline.createInterface definitions were also updated.

Parser:

#7471 Adds support for parsing of BigInt (Arbitrary precision integers) (thanks @goodmind)

0.96.1

Object literals with spreads can be described by object types with spreads in the signature verifier/builder.

0.96.0

Likely to cause new Flow errors:

Recently the Object and Function types changed their meaning from "any function type" to "any type." Accordingly, various Object and Function annotations that made sense before this change have been updated in various library definitions.
Various other PRs making improvements in library definitions have been merged in this release. These include core definitions like Date and Object as well as other DOM and Node definitions.
We now issue a error when a value that is clearly not a type could be exported as such.
We now issue an error when a function is imported as a type.

Notable bug fixes:

Some commands are not expected to update server state. But if such a command is cancelled and we run a recheck before rerunning the command, not updating the server state would make it seem like that recheck never happened (and lead to spurious future rechecks). This has now been fixed.
Fixed node_modules filter for lint warnings, which didn't work on Windows, and didn't respect the node_resolver_dirnames config option.

Misc:

Results of batch-coverage in lazy mode can be misleading, as it does not account for the fact that the currently checked files might not be the ones the user is querying for. Running batch-coverage in lazy mode is now disallowed.
Fixed an issue with flow lsp where logs would not be flushed.

0.95.2

The inferred statics object type of React.createClass({}) will contain defaultProps: void, instead of defaultProps: {||} (unsealed empty object).
Bug fix in internal cache mechanism

0.95.1

Added an overload for JSON.stringify allowing mixed input, which returns string | void. Without this, you can't call JSON.stringify on a mixed value at all, because while Flow does allow refining mixed to "not void" (e.g. x === undefined ? undefined : JSON.stringify(x)), it does not support refining mixed to "not a function" (e.g. imagine you could do x === undefined || typeof x == 'function' ? undefined : JSON.stringify(x)). This rolls back some of the more restrictive behavior introduced in v0.95.0, but is still more restrictive and more accurate than in <= v0.94.0.

0.95.0

Likely to cause new Flow errors:

Disallow undefined and functions in JSON.stringify: JSON.stringify(undefined) returns undefined instead of string. Rather than make it always return string | void, or use overloads to return void on those inputs, we instead disallow those inputs since they are rarely the intended behavior. (#7447)

New features:

flow batch-coverage: A new command to compute aggregate coverage for directories and file lists. Instead of producing the coverage at each location, it sums them and reports the per-file percentage and the aggregate percentage.

Bug fixes:

Fixed incorrect reporting of signature verification lint errors in unchecked files

Other improvements:

#7459 Add type for Symbol.prototype.description (thanks @dnalborczyk)
#7452 Add types for String.prototype.trimStart/trimEnd (thanks @dnalborczyk)
#7500 The "kind" of an autocomplete result is now reported over the Language Server Protocol, improving the autocomplete UI (thanks @vicapow)

0.94.0

Bug fixes:

Fixed dynamic-exports lint's spurious errors on exported classes and functions
Handle package.json files that are valid JSON but invalid packages

Performance:

Reduce memory usage by filtering suppressed lint errors before formatting the errors for printing
Quicker responses to cancellation requests

Many libdef fixes and other improvements from the open source community:

#3209 Fix autocomplete for generic type aliases (thanks @vkurchatkin!)
#6750 Remove shadowed generics in Proxy$traps (thanks @talbenari1!)
#6000 Document async function return type (thanks @callumlocke!)
#7448 Tweaks to built-in http module (thanks @STRML!)
#4570 Update types for Web Audio API (thanks @fand!)
#5836 Fix examples in libdefs/creation page (thanks @tomasz-sodzawiczny!)

Additional lib def improvements:

Make current write-only in React.Ref - allowing union types for ref
Add setMediaKeys API to definition of HTMLMediaElement
Make type parameter to http$Agent covariant

0.93.0

Likely to cause new Flow errors:

Removed a constraint involving any types and React proptypes for efficiency. This may result in some errors no longer being reported.

New Features:

A new lint (dynamic-export) which when enabled will warn when most dynamic types are exported from a file.
Flow now distinguishes between any and empty when computing line coverage. empty types are colored blue and any types red when using the --color option. Note that this may cause new expressions to be considered uncovered.

Notable bug fixes:

Fixed a non-termination condition during this-substitution.
Fixed an issue where inexact-spread lint errors could appear in the wrong position.

Many, many libdef fixes and improvements! Many thanks to the open source community for these, and to @nmote and @jbrown215 for reviewing and merging so many of these!

#4388 add missing InputEvent(Listener|Handler|Types|) (thanks @keithamus!)
#4664 Fix IntersectionObserver constructor definition (thanks @apostolos!)
#4858 Make ServiceWorkerMessageEvent extend ExtendableEvent (thanks @keyiiiii!)
#4772 add indexer property to string lib def (thanks @zacharygolba!)
#5529 Add module.builtinModules to core libdef (thanks @simenB!)
#5574 fix return parameter for writable.setDefaultEncoding() (thanks @dnalborczyk!)
#5578 add util.callbackify to node type def (thanks @dnalborczyk!)
#5628 Add lib declaration for BroadcastChannel (thanks @schmatz!)
#5866 Add definition for timingSafeEqual() (thanks @rolftimmermans!)
#5988 add destroy method to streams (thanks @hiikezoe!)
#6091 Fix static declarations for XMLHttpRequest (thanks @robin-pham!)
#6339 Fix parent of AnimationEvent (thanks @ngyikp!)
#6367 Add types for Object.getOwnPropertyDescriptors (thanks @disnet!)
#6471 Actualize node's EventEmitter API definition (thanks @antongolub!)
#6535 add Element.prototype.toggleAttribute (thanks @keithamus!)
#6614 Add TransitionEvent to dom libdef (thanks @koddsson!)
#6785 Allow specifying encoding as string in options field of appendFile, appendFileSync (thanks @cappslock!)
#6963 Add Audio declaration (thanks @vldvel!)
#7011 Use more specific type for navigator.serviceWorker (thanks @dhui!)
#7097 Add type definitions for message events (thanks @wachino!)
#7122 Support for Uint8Array (thanks @cakoose!)
#7144 Updated URL modules definitions for Node.js 10 (thanks @MrFranke!)
#7146 Fix type definition of Node.js dns.lookup() (thanks @shuhei!)
#7215 fix https interfaces (thanks @cakoose!)
#7225 make createContextualFragment return a DocumentFragment (thanks @wincent!)
#7342 add Document.queryCommandSupported (thanks @Eazymov!)
#7358 Add oncontextmenu to HTMLElement (thanks @jasonLaster!)
#7363 Add MediaDeviceInfo declaration (thanks @ea167!)
#7367 Add userSelect to CSS declaration (thanks @shubhodeep9!)
#7368 Fix fs.promises.readFile being incorrectly overloaded (thanks @Macil!)
#7381 add EventSource to dom libdef. Likely to cause new errors (thanks @SlIdE42!)
#7386 fix XDomainRequest in bom libdef. Likely to cause new errors (thanks @Mouvedia!)
#7387 Added optional displayName property to React$Context (thanks @bvaughn!)
#7405 Basic support for typeof x === 'symbol' (thanks @mroch!)
#7420, #7430 and #7440 Various React improvements (thanks @threepointone!)
#7423 make useRef type non-nullable (thanks @trysound!)
#7445 add Stream type to Node thanks (thanks @goodmind!)

Misc:

Updated our website and GitHub issue template to make it easier for open source users to contribute to Flow!
Various improvements to the AST, including the differ, typed AST and the AST mapper

0.92.1

Notable bug fixes:

Fixed a bug introduced in v0.92.0 which could cause the server to crash when using an IDE.
Fixed module.system.haste.name_reducers option for Windows (#7419 - thanks jamesisaac)

0.92.0

Likely to cause new Flow errors:

Some fixes to tagged template literals will surface a new set of pre-existing errors!

New Features:

This release culminates months of hard work on quality of life improvements for IDE support. Expect your requests to be faster, and your requests to take a bit less time.

Several Flow commands can run in parallel now, i.e. you can still get type definitions while rechecking. Big props to @glevi for this massive QoL improvement!
Alongside this change, @glevi also released a new lazy-check mode that leverages watchman to reduce the number of checked files. Learn about it in the docs.

Notable bug fixes:

Thank you to the opensource community for these fixes!

#7354 Fix MouseEvent type definitions
#7262 Update types for WeakMap
#7100 Add missing crypto.randomFillSync and crypto.randomFill methods
#7356 Add definitions for new debug hook useDebugValue
#7352 Rename React hook useImperativeMethods -> useImperativeHandle
#5665 Fix arity of clearInterval, clearTimeout
React.memo now accepts any kind of Component

0.91.0

Likely to cause new Flow errors:

Better positioning for React error messages. This may move already existing (and suppressed) errors to new positions.

New Features:

The --lazy-mode flag was added to all commands that may autostart a server

Notable bug fixes:

Fix a crash when using private class fields (fixes #7355)

Misc:

Various additions and typo fixes on flow.org docs and README (thanks @fschindler, @dominicfraser, @keithamus, and @fterh)
Misc OCaml tweaks (thanks @rvantonder)
Huge reduction (~15%) in total memory usage (thanks @nmote)
Huge reduction in error collation time (thanks @panagosg7)

0.90.0

Likely to cause new Flow errors:

Removed unsafe rule allowing Date instances to be used as a number
Changed $Shape<> types to reject null and void as subtypes
Removed unsafe refinement from mixed to a function type using typeof

Pull Requests:

#7290 Add support for Path2D constructor arguments (thanks @zpao!)
#7221 use package that works with 7.x in babel doc (thanks @rob2d!)
#7231 Improve type of 'mkdir' and 'mkdirSync' (thanks @mrtnzlml!)
#7278 fix: update toc links in hoc pages (thanks @evenchange4!)

Notable bug fixes:

Fixed issue where errors involving $Shape<> types were positioned poorly

Parser:

Fixed decoding of html entities at beginning of JSX children
Fixed offset calculation in estree output to account for multibyte characters

0.89.0

Likely to cause new Flow errors:

Big revamp to React typing with the goal of adding support for React.forwardRef and better typing higher-order components. Docs are available here.

New Features:

New deprecated-utility lint complains about deprecated Flow types. To start off, $Supertype and $Subtype are now deprecated. They were unsound and can usually be replaced using shapes or bounded generics.
React.AbstractComponent makes it way easier to type React higher-order components.
React.Config is also intended to help type React higher-order components.

Notable bug fixes:

flow coverage --color handles multi-byte characters better
flow coverage now supports --strip-root

Misc:

We've deleted flow gen-flow-files due to bitrot. We do plan on building a better version in the future.
Various libdef updates. Thanks for all the PRs!
{} now consistently represents an unsealed object. You can read more in the documentation. example

0.88.0

Likely to cause new Flow errors:

Made Function and Object types be aliases for any. They were always unsafe types, just like any, but they had peculiar behavior. This change revealed places where they were handled improperly within Flow, and ended up surfacing type errors that were previously missed.

New Features:

Added the experimental Watchman lazy mode (flow start --lazy-mode watchman). This improves the lazy mode experience for repositories which use Mercurial and Watchman. We will document it more when/if it proves itself.
Added flow config check which validates the .flowconfig.

Misc:

Made miscellaneous improvements to the AST differ, which improves the output of global rename.
Made .flowconfig parsing less strict (in particular, if the --ignore-version flag is passed, do not fatal on unrecognized config options).
Performed a code cleanup in type normalization that caused some types in type-at-pos to be displayed differently.
Removed redundant information in stored ASTs resulting in a modest reduction in memory usage.
Flow assigns long string literals type string, rather than the singleton type of that literal. Now, this fact is surfaced in error messages.
Fixed stack overflows:
- When checking a large number of files.
- When a large number of errors are present.

Libdefs:

Added React.Suspense.
Removed React.useMutationEffect hook.

0.87.0

Likely to cause new Flow errors:

Fixed an error in the React.createRef definition. Refs are for any type of value, not just React elements.

New Features:

Added support for subcommands to the CLI and a flow config find command that finds the .flowconfig governing path.

Notable bug fixes:

Fixed file_sig for deep destructured requires.
Fixed a stack overflow in the flow ls --json command.
Fixed a crash when the module reference prefix is used on an untyped module.

Misc:

Removed the flow port command.
Various improvements to the AST differ.
Made progress in shifting from concrete to abstracting locations, by
- functorizing a number of modules over their use of locations, and
- updating the core typechecking logic to operate solely on structures containing abstract locations.
Removed the redundant Expression.Member.computed field from the AST.
Allow configuring path to node in runtests.sh.
Refactored JSX.frag_closingElement of the AST to not be an option.
Fixed error localization in import type and import typeof with default.
Added the location of function signatures to the AST.
Removed prototype members when autocompleting JSX props.
Cleaned-up handling of the mixed case from LookupT.
Optimized the case of enumerations when used as keys.

Library definition changes:

Made geolocation PositionOptions optional.
Added flow definitions for React hooks.
Added a calculateChangedBits parameter to the React.createContext definition.
Added React.ConcurrentMode and React.StrictMode definitions.
Added React.lazy and React.memo definitions.

Parser:

Refactored flowconfig option parsing into a list of parsers.

0.86.0

Likely to cause new Flow errors:

New errors may arise from generic definitions which lack the necessary annotations. Flow does not infer generic types, and the types it used to infer in their place occasionally masked errors. The types it now infers are still not generic, but will mask fewer errors.
Fixed bug in union type checking which caused Flow to miss some errors when used in conjunction with generics and certain utility types.
Improvements to constraints involving any may result in new errors.

New Features:

type-at-pos can reconstruct spread objects when evaluation of the spread is not possible.
type-at-pos now supports implicit instantiation with _.
Added core types $ReadOnlyMap, $ReadOnlyWeakMap, $ReadOnlySet, and $ReadOnlyWeakSet. These types and their mutable counterparts mostly follow the pattern of $ReadOnlyArray and Array: the read-only parent class provides non-mutating methods, and the usual mutable class extends it. Unlike $ReadOnlyArray, the new types are invariant in their type parameters.
Added the React.StrictMode type.
Added the flowtest package. flowtest is a CLI tool for running tests on Flow types.
Added the sharedmemory.heap_size option.

Notable bug fixes:

Restructured file signatures to support overloading in exports.
Allow named exports in exact module.exports objects.
Fixed forward references for declare function.
Various bug fixes to the type normalizer and to type-at-pos.
flow suggest no longer outputs <<anonymous class>>.
Imported symbols should no longer appear Remote in type-at-pos.

Misc:

Improved formatting in JavaScript output.
Improved the way the element type of an array is determined.
Various improvements to the AST differ.
WeakMap keys must now be objects.
Improved profiling for error collation and formatting.
When a union lower bound flows into a union upper bound, if both unions are enums, we use the underlying set representation to handle this in O(n log n) time instead of quadratic time.
The type normalizer properly represents utility types instead of treating them as generics.
The type normalizer provides more accurate information about generics.
Flowing a string lower bound into a union upper bound occurs in O(log n) instead of O(n) when the union is an enum.
CJS modules' namespace objects are now treated as covariant.
The type normalizer reconstructs literal types more precisely when preserve_inferred_literal_types is set. This does not change the behavior of type-at-pos.
Deleted the experimental.cancelable_rechecks option.

Parser:

Removed the deprecated expression field from Function nodes in the AST.
Enabled some tail call optimizations in flow_parser.js which should cause it to stack overflow in fewer cases.
The layout generator no longer prints empty statements as {} instead of ; in pretty mode.
Allow anonymous function parameter types inside generics inside arrow function return types. For example, we disallow var x = (): (string) => number => 123 because the first => is ambiguous. However, var x = (): T<(string) => number> => 123 is not ambiguous and we no longer disallow it.

0.85.0

Likely to cause new Flow errors:

Fixed an issue that caused missing annotations errors to be suppressed.

Please read the full post for more information on what changed and tips on dealing with the new errors.

0.84.0

Likely to cause new Flow errors:

Earlier, type constraints between any and other types would be dropped. Instead, they are now propagated. In some cases, this unblocks further constraint solving and helps find more errors.
When a variable is equality-checked with a literal, the variable's type is refined. Earlier, if the variable's type was incompatible with the literal's type, it would silently be refined to empty, whereas now this is an error.

New Features:

Added support for wildcard (_) type arguments to function / constructor calls. This is especially useful when some type arguments are sufficient to pin down the type of the result; the others can simply be _.

Notable bug fixes:

Fixed a case that would crash get-def and find-refs
Fixed a bug with unreachability analysis for ternary expressions
Fixed a bug with refinements merging at the end of switch statements

Misc:

Fixed various AST printing / layout bugs
Made various improvements to the AST differ
Refactored parts of get-def and find-refs to reuse code
Made progress on abstracting locations in the core type inference engine
Made progress on module signature verification and generation
Merged PRs that improve type declarations: e.g., getBoundingClient returns a DOMRect instead of a ClientRect.

Parser:

Support for ... to indicate inexactness in object types

0.83.0

Likely to cause new Flow errors:

Fixed a bug where type precision was lost at module boundaries

Notable bug fixes:

Prevented an exponential blowup when union types flow into "maybe" (?T) or "optional" (essentially T | void) types
Allowed {p:T} to be a subtype of $Shape<{+p:T}>

Misc:

Fixed exception when using --traces
Changed --verbose to not log while loading flowlibs by default; pass --verbose-flowlibs to override
Added ability for LSP clients to cancel previously-sent commands
Improved location of diagnostics via LSP while typing
Fixed LSP textDocument/definition response when there are no results (prevents bogus jump-to-definition inside comments, for example)
Limited number of errors returned via LSP to 200, to improve Nuclide and Visual Studio performance
Fixed an exception when attempting to focus a file that Flow ignored

Library definition changes:

Added ResizeObserver declarations
Added missing net$Server.listening property
Added process.cpuUsage()

Parser:

Fixed export default async function to be a FunctionDeclaration not a FunctionExpression
Made instance properties named 'constructor' a syntax error

0.82.0

Likely to cause new Flow errors:

Removed the ability to use functions as type annotations. This ability was originally designed to support ES3-style classes, but in practice causes confusion and missed errors. We found that the vast majority of added errors were detecting legitimate bugs, and the remainder could be easily updated to ES6 classes.

New Features:

Added worker utilization and GC profiling under the --profile mode.

Misc:

Further improved performance of IDE requests on large codebases.
Tweaked OCaml GC settings to reduce time spent collecting on workers.
Improved performance and reliability of watchman integration.

0.81.0

Likely to cause new Flow errors:

Fixed a bug which allowed refinements to incorrectly escape outside of conditional expressions.
Plugged a hole which incorrectly allowed this to appear in a constructor before a super call in some cases.
Some errors related to uses of typeof may be reported at a different location than they previously were.
Disallowed shadowing class methods with incompatible properties of the same name.
Made miscellaneous other bug fixes that manifest only in rare cases.
Made module properties covariant, meaning that e.g. mutating properties on the object returned by require() is not allowed.

Notable bug fixes:

Improved performance of IDE requests (e.g. autocomplete, get-def) on large codebases roughly 3x.

Misc:

Made a number of improvements to the diffs generated by global rename.
Changed how saved-states are loaded.
Added additional information to error messages for missing annotations for type parameters.
Disallowed class static properties named prototype or constructor.

Library definition changes:

Added type definitions for pointer events.
Updated DOM scrollIntoView to match latest spec.
Updated String.prototype.toLocaleLowerCase and toLocaleUpperCase to allow optional locale parameter.
Made Object.prototype.valueOf return mixed instead of Object.
Removed the indexer from the Object libdef.
Updated TextDecoder to properly accept a BufferSource.
Added MediaStreamTrackEvent definition.
Made String.match return RegExp$matchResult.
Added additional constants to the fs module.

0.80.0

Likely to cause new Flow errors:

We've changed how Flow lint severity is calculated. We used to look at every location mentioned by a lint and we'd use the lowest severity, where Off < Warn < Error. Now we just use the severity at the lint's first location. Flow suppression comments (e.g. // $FlowFixMe) can still suppress a lint at any mentioned location. Depending on your setup, this change may expose Flow lints which were turned off by accident. For example, a "Sketchy Null Check" lint in a file with that lint set to error would have been turned off if it also mentions a file with that lint off. Now that lint will show up as an error.
Some libdef changes may cause a few errors. For example, Headers.get and URLSearchParams.get are now annotated to return null | string instead of string and File.lastModifiedDate: any was replaced with File.lastModified: number.

Notable bug fixes:

Fixed a crash that could happen when a persistent connection disappears.

Misc:

Many improvements to the libdefs and docs. Thanks for the PRs, everyone!
Small perf improvement by making BoundT (internal representation of type parameter bounds) smaller.

Parser:

Small change to the OCaml AST for the extends expression in a class declaration. Moved a couple of properties to their own node.

0.79.1

Notable bug fixes:

If a server was not already started, running flow status --flowconfig-name 'name' would start a server without taking into account the new flowconfig name. The same is true for running flow without any command specified. This changes this by passing the name to the new server.

0.79.0

Likely to cause new Flow errors:

A fix in requiring annotations on exports: Flow uses type variables for unannotated program parts. To facilitate inference, these type variables may not appear in input positions of exports. Before, Flow would not complain if a type variable appeared in both an output and an input position of an export (in that order), as the first occurrence marked the use of the type variable as legitimate. Now, Flow analyzes exports in a polarity-sensitive way and will require an annotation in the above scenario.

New Features:

Saved state:
- Added an initialization path, where instead of parsing every file dependency, it loads the expected results for each available file from the saved state.
- A hash is stored for each file in the saved state to determine whether it has been modified and therefore needs to be reparsed after the state is loaded. This way, saved state servers can skip parsing files that are unchanged.
- If loading saved state fails, Flow falls back to a full initialization, unless --saved-state-no-fallback is passed in which case Flow fails.
- Saved state tests are included in the automated testing framework.
Profiling was refactored to support hierarchical profiling. That is:
- Support running timers inside of other timers.
- Support merging a finished profiling object (like from a recheck) into the currently running profiling object (like from handling a command).
Cancelable workloads (jobs that handle commands). If Flow determines that a file has changed while handling a command, it stops, performs a recheck, and then re-runs the workload from scratch.
The Flow diff checker became more fine-grained, by including comparisons at the level of loops and variable declarations.
Added a name field in the [options] portion of .flowconfig.

Notable bug fixes:

Send the server logging context to the LSP command and use that when logging success and failure events instead of the LSP command's context (which is logged as the client context).
Fixed stack overflow when running flow --json with a large error output, by moving to a tail-recursive implementation.
Fixed string literal printing in flow suggest.

Misc:

Library definition improvements for React$Context.Provider and CanvasRenderingContext2D.imageSmoothingQuality.
Typed AST: The constraint generation phase returns a version of the AST that includes a type for each program node.
Added doc details for $Shape.
The changes in saved state were followed by code refactorings in various checking modules (types_js.ml, merge_service.ml, rechecker.ml, etc.).

0.78.0

New Features:

Added the unnecessary-invariant lint, which will flag uses of invariant when Flow can determine that the predicate must be truthy.
find-refs now traverses union types.
The --flowconfig-name flag now allows you to specify the name of the flowconfig.
Added a file_watcher option for .flowconfig.
Rechecks will now stop and restart as soon as Flow notices other file changes. The previous behavior was to finish the recheck and then start a new recheck. The new behavior is currently off by default and can be enabled by setting experimental.cancelable_rechecks=true in .flowconfig. Note: This feature is experimental. When it stabilizes, the flag will be removed and this feature will be enabled by default.

Notable bug fixes:

flow-upgrade should no longer trigger ENOENT on Windows.
Running Flow with --profile should no longer throw on Windows.
Suppression comments preceding lint errors for disabled lints should now be correctly considered unused and trigger unused suppression warnings.
ArrayBuffer's static indexer property is now correctly marked covariant. This should prevent errors like this one.
Fixed a bug in which the polarity of a type parameter was propagated to all subsequent type parameters, causing errors when covariant type parameters followed contravariant or invariant ones. This bug is demonstrated by the difference in behavior of the following examples:
- Incorrect
- Correct

Misc:

Flow now prints array types as Array<T> instead of T[] in JavaScript output.
Flow now prints {foo:x} as { foo: x } in JavaScript output, as Prettier does.
Library definition changes:
- Added destroyed to net$Socket.
- Added document.scrollingElement.
- Added dgram$Socket$rinfo to the Node library definitions.
- Added WebAssembly library definitions.
- Added the groups property to RegExp$matchResult.

0.77.0

New Features:

New lint to disallow a && b when a is a number. See https://flow.org/en/docs/linting/rule-reference/#toc-sketchy-number
Support multi-hop/indirect find-refs in LSP
Expose renaming through the Flow LSP

Notable bug fixes:

Fix server crash on persistent-command unhandled exception

Misc:

Library type definition changes: created explicit type for Context, added readAsBinaryString to FileReader
AST differ utility to power rename
More uniform bucketing of jobs to be run in parallel leads to higher CPU utilization
Improvements to type printing
Treat getters and setters as proto fields

Parser:

Optimization when saving comments

0.76.0

Likely to cause new Flow errors:

Flow now only allows subtyping polymorphic types with the same number of type parameters. This added strictness lets Flow avoid some slow typing code and shouldn't cause too many problems.

New Features:

New [declarations] section in the .flowconfig which can suppress all errors in 1 or more files. Thanks @LegNeato! (#4916)
Added flow server --file-watcher watchman flag which uses Watchman for file watching instead of the builtin file-watcher, dfind
Added a --expand-type-aliases flag to type-at-pos. It replaces type aliases with their bodies in the returned type.
Added flow save-state command to generate a saved state file (experimental & loading a saved state isn't ready yet)

Notable bug fixes:

Fixed type-at-pos for B in import type { A as B } from 'M';
Fixed find-refs, autocomplete, get-def, etc for $Facebookism$Idx

Misc:

Bunch of improvements to the libdefs. Thanks everyone for the PRs!
Handful of improvements to the type normalizer, which improves the type-at-pos output
Tweaked error messages for + operator to match other binary operators

0.75.0

Likely to cause new Flow errors:

Added return type to RegExp.prototype.match() (thanks @peter-leonov!)
Fixed callable property lookup rules to no longer walk the prototype chain
Made it an error to import a type via destructuring on a require call (use import type instead)
Made the type of Array.of stricter (thanks @wchargin!)

New Features:

Added typing rules to support {| |} as a subtype of { +p?: T }
Added support for optional catch bindings (thanks @cpojer!)
Improved performance of enum equality checks
Added support for [[call]] syntax in objects and interfaces
Many improvements to flow find-refs command
Many improvements to LSP support

Notable bug fixes:

Fixed crash when refining a non-value

Misc:

Updated node.js libdefs (thanks @hcz!)
Fixed some flakiness in the LSP tests
Added libdefs for ECDH class (thanks @eleith!)
Added --check flag to flow ast command
Added filter property to CanvasRenderingContext2D libdef
Added timingSafeEqual method to node crypto module (thanks @n-johnson!)

Parser:

Added Decorator node, matching estree

0.74.0

Likely to cause new Flow errors:

Flow now performs exactness checks that were previously skipped in some cases (e.g when $ObjMap is used on an exact type).

New Features:

Significant progress towards supporting the LSP (Language Server Protocol) natively.
Allow custom module resolvers (experimental, only works with the haste resolver system).
Flow lints for unnecessary optional chaining.

Notable bug fixes:

Allow autocomplete to work with optional chaining.
Only report status as GCing when actually GCing.

Misc:

Improve the args used when launching child processes to give more information about the process when you run ps.
Add a max_literal_length option to .flowconfig which replaces a magic constant.
Multi-hop (indirect) find-refs now supports class instances.
Revamp find-refs on ES6 named exports to improve reliability.
Allow internal slot properties to be optional.
Improve shutdown of the server monitor process.
Try to gracefully stop server process when monitor exits.
Print server status updates on new lines when Flow is not called from a tty.
Add additional properties to the react-dom/server libdef.
Add 'as' property to the HTMLLinkElement libdef.
Add a libdef for the Asynchronous Clipboard API.
Update libdef for Clients.matchAll to return an Array rather than an Iterator.

Parser:

Rename InterfaceType to InterfaceTypeAnnotation.

0.73.0

Likely to cause new Flow errors:

New Features:

Inline interface types (var foo: interface { x: string } = bar;). Will be particularly useful in the future when we revamp object types

Notable bug fixes:

flow find-refs threw an exception when used with an unchecked file. Now it treats the file as if it were checked, like other single-file commands do
flow type-at-pos now returns the class name for declare class declarations.

Misc:

Added .mjs to the list of extensions that Flow reads by default
Perf improvements to calculating the dependency graph during recheck. Should help recheck perf on large repositories.
flow find-refs --multi-hop now parallelizes the work and is much faster
Support using flow find-refs and flow get-def with destructuring patterns as a starting location
Support using flow find-refs with default (as in export default ...) as a starting location
Bunch of small fixes where flow find-refs would miss certain locations
Tweaked the location returned for flow get-def when used for default and named imports
Lots of libdef updates. Thanks for the PRs!

Parser:

Inline interface type support

0.72.0

Likely to cause new Flow errors:

We've made explicit the order in which imports are merged into a module during typechecking. This fixes an edge case in which lazy modes and non-lazy modes would report different errors. This may change the order in which code is typechecked, and therefore may expose errors that Flow previously missed.
Treat $Exact<empty> as empty. Before, ({p:0}:$Exact<empty>) was not an error due to missing ground subtyping and decomposition cases. It is now an error.
The $Either<T>, $All<T> and $Type<T> types are not supported any more.

New Features:

Find-refs now includes references to all types related to an object literal through subtyping, and a multi-hop mode was added that determines when object types are related through subtyping and links them.
Work towards the new object model:
- Ensure fields overwrite methods in interface definitions (since properties there are flat).
- Store proto fields in a separate map than the one used for own properties in classes.
- Declare Function.prototype.bind as a proto field.
New/call can now be passed explicit type arguments for polymorphic instantiation. Currently this is supported by the Flow parser. Babylon support does not exist, but it is planned.
Made * a deprecated type, under the deprecated-type strict flag.
Added support for hover, completion and error reporting (publishDiagnostics) to flow LSP.
Implemented nullish coalescing as per the TC39 proposal.
Added a debug flag --expand-json-output to print an extended JSON output for type-at-pos.
Updates in typings:
- Added the definition for onclose to the IDBDatabase interface in lib/indexeddb.js (reference).
- Added onmessageerror to Worker interface and fixed type of MessagePort.onmessage in lib/bom.js.
- Added a swap64() to the Buffer type and a Buffer property in lib/node.js.
- Added Intl objects for the built-in JS language API (ECMA-402 - Internationalization API).
- Added tuple types to WebGL uniform**v setters in lib/dom.js.

Notable bug fixes:

LSP: Fixed races in reporting exit status over persistent connection and in test.
Fixed error reporting when accessing statics and simplified error localization.

Misc:

Added documentation for Object type, "Flow for Atom IDE" in Atom plugins, Flow Strict and the nonstrict-import lint rule.
LSP supports file edits. Each client now stores the content of files that are opened.
Added LSP test checking that contents of open files persist after restart.
Removed Travis from CIs.
Type normalizer: added option to flag cases where the inferred type parameter is shadowed by another parameter with the same name, and fixed support for recursive polymorphic types.
Removed dependency on ocp-build (windows now uses ocamlbuild).
Introduced a union-find/disjoint-set data structure used in find-refs to maintain sets of related types.
Fixed return types for WebGLRenderingContext#is* methods to all return booleans.
Rearranged contents of src/server directory.
Refactored find-refs by splitting variable and property handling in separate files, and breaking down functions based on their purpose (e.g. local vs global).
Made $Subtype and $Supertype "unclear" types when not in a library context.
type-at-pos now prints types at the client-side.
Minimum OCaml version is increased to 4.05.0.
Avoid redundant substitution by caching the result of substituting a polymorphic definition with a list of type args.

Parser:

Added support for nullish coalescing ?? (as above).
Simplified object type parsing. Dangling + or static inside an object type (e.g. {+}) are now disallowed.
Added support for a proto modifier in declare class, to specify that the property is a prototype property instead of a class one.
Internal slot properties in object types (e.g. [[new]]).
Explicit type arguments in new and call expressions, e.g., f<T>(x).
Allow reserved words as optional chain property names.

0.71.0

Likely to cause new Flow errors:

The result of the unary minus (negation) operator would sometimes be incorrectly generalized to number rather than a literal type. This has been fixed.
Further restrictions on module and exports. The disallowed patterns include computed property accesses as well as clobbering or aliasing module and/or exports.
- These restrictions are only enabled if experimental.well_formed_exports=true is specified in .flowconfig.

New Features:

Fragment has been added to the default exports of the React libdef.
Invoking find-refs on a property key in a type declaration will now also yield that key in object literals.
Files can now be marked @flow strict-local. This mode is the same as @flow strict, except it does not require dependencies to also be strict. @flow strict is still the recommended mode, but @flow strict-local allows enabling strict checking for files before all dependencies can be made strict. Once all the dependencies are strict, a @flow strict-local file can be upgraded to a @flow strict file. A @flow strict file cannot depend on a @flow strict-local file.
Type support for the Stage 1 Optional Chaining proposal. To use this feature, set esproposal.optional_chaining=enable in your .flowconfig.
- Note: We currently disallow calls whose callees are member expressions when the call or member is part of an optional chain. This restriction will be lifted in the future when we reconcile the optional chaining implementation with our implementation of method calls.

Notable bug fixes:

The type normalizer now correctly tracks the scope of type parameters instead of assuming they share the scope of the type definition.
Test output files are cleared before running tests. This prevents old errors from being printed on subsequent failing runs when those runs do not produce errors.
estree_translator properly handles all cases of the DeclareClass AST node.
The suggest command has been fixed to work with the new type normalizer.
When evaluating spreads inside array literals, we determine the element type early, preventing exponential complexity and infinite loops.
Object spread in JSX props now preserves exactness.

Misc:

Various improvements to find-refs.
Optimizations for polymorphic subtyping.
Adds CircleCI for continuous integration and migrates the flow.org build from Travis to Circle.
New tests for LSP support (disabled by default).
Certain exceptions will now print backtraces in addition to the exception string.
Improvements to spreading in array literals.
Support for Flow coverage reports using codecov.io.

Parser:

The AST has been updated to use snake_case for record fields rather than camelCase. Some field names have also been updated.
Support has been added for the Numeric Separators proposal, currently Stage 3.
The AST representation for Optional Chaining has been updated to use new OptionalMember and OptionalCall nodes instead of the existing Member and Call nodes, and parentheses now correctly limit the scope of short-circuiting. This reflects the current Babel implementation.

0.70.0

Likely to cause new Flow errors:

Existing Promise libdefs for the cases where null or void is passed as callbacks to then and catch caused us to miss type errors. These libdefs have now been made stricter.
Spreading an object of exact type used to result in an unsealed object type, which allowed adding new properties not mentioned in the original type. Now, the result has an exact object type, which bans adding new properties.
New missing annotation errors might be reported. These requirements were missed before because of implementation bugs. We now use the polarity-sensitive type visitor to walk exported types, which reports errors in the expected places. There are several exceptions, which can be exported without annotations: e.g., object and array literals; initialized class properties (instance & static); and this parameters.
Interactions of typeof with speculative typechecking would lead to missed errors, which will now be reported as expected.
Various new restrictions now ban abuses of module and exports.

API changes:

The output format with --json-version 2 includes a breaking change, affecting the "context" property in every location. Previously it was just the first line of the context, which made printing multi-line errors tricky. Now it includes up to 5 lines of context.
New version of ocaml-sourcemaps. The API changed slightly, but most of the changes are adding support for reading/composing sourcemaps, which we aren't using.

New Features:

Updated React libdefs with the new createContext and createRef APIs introduced in React 16.3.
Classes can now be spread (copying over their static properties).

Notable bug fixes:

Lazy_mode_utils.focus_and_check wasn't filtering out files properly when processing updates.
The monitor would keep restarting segfaulting servers.
Lint errors would interfere with speculative typechecking (with union / intersection types)
Switch to using nonblocking file descriptors with Lwt to address Flow hangs
Fixed a crash in the no-color error printer caused by degenerate locations.
Improved refinement of truthy/falsy intersections.

Misc:

Various improvements to type-at-pos and friends, including converting normalized types back to AST nodes for printing.
Various improvements to find-refs.
Various CI improvements as part of the move to Circle.
Various debugging utils.
Array types are now printed as Array<T> instead of T[].

Parser:

Exposed implements and mixins fields of the DeclareClass AST node.
Added tokens option to JS API: flow.parse(..., { tokens: true }) will now return the token stream, like flow ast --tokens does.
Added options support to the parser's C interface. This change lets you pass a map of options to the parser via the C++ API, and from JS via flow-parser-bin. You could already do this from the js_of_ocaml parser, so now their APIs match.

0.69.0

Notable bug fixes:

Fixed a couple bugs in pretty printing errors which could cause flow status to throw an exception

Misc:

Lots of internal refactoring/changes this release, so not too much to report!
A bunch of libdef improvements. Thanks everyone for the PRs!

0.68.0

Likely to cause new Flow errors:

Previously, Flow would allow you to write if (foo.unknownProp) { ... }.

Now Flow disallows testing unknown properties in conditionals. If foo is a union type like { x: string } | { y: number }, x and y are known properties and z would be an unknown property

New Features:

Improve union optimizations
Add type declarations for new React 16.3 lifecycle methods

Notable bug fixes:

Fix get-def for declare var
Fix type-at-pos for opaque types

Misc:

Remove special-casing of declare var exports, long deprecated

0.67.1

Restore accidentally-deleted Object.setPrototypeOf library definition.

0.67.0

Likely to cause new Flow errors:

Replace some any-typed API definitions with actual types. This may cause errors in code that uses those APIs.

New Features:

find-refs now has support for object type properties.

Notable bug fixes:

find-refs can now find identifiers used as JSX component classes (e.g. <Foo/>).
Fix nontermination that could occur when printing types (e.g. for type-at-pos).

Misc:

Fix type-at-pos on method calls that have been affected by type refinements.
Add --profile flag to flow force-recheck.
Fix --retry-if-init false.
Improve type-at-pos location for opaque type declarations.
Add a message to the flow status output when the server is in lazy mode.
Include filename in flow ast output.
Add typings for ReactDOM.hydrate().
Make process.umask's argument optional.
Some miscellaneous improvements to code quality.

Parser:

Optional chaining parser support. This feature does not yet have type system support and should not be used.

0.66.0

New Features:

Error message redesign to help you better debug typing issues in your programs.
Upgrade global find-refs from experimental to beta.
Improve global find-refs performance by 2x-20x depending on the workload.
Support for CommonJS imports/exports with global find-refs.
Find usages of overridden instance methods with find-refs.
Improvements to type reporting services like type-at-pos and coverage thanks to a type "normalizer" rewrite. Notably, the normalizer and client services like type-at-pos now:
- Make more aggressive use of type aliases, leading to more compact results,
- Correctly distinguish between class types and their instance counterparts,
- Report abstract type parameters themselves instead of their bounds, and
- Precisely report recursive types.

Likely to cause new Flow errors:

Unresolved type variables internally unified with any no longer completely resolve to any. This may uncover new bugs in your programs where some inferred types silently resolved to any.
Fix type system "stall" for spreads of null and undefined. This meant programs like ({...null}: {p: number}); incorrectly passed Flow. If you were spreading a maybe-object type then Flow may catch some new bugs in your programs.
Because of the new error messages some suppression comments (configured with suppress_comment) will become unused since error locations moved.

Misc:

3.5x performance improvement for some Facebook projects that makes heavy use of complicated unions of string literals such as GraphQL enums generated by Relay. (Performance improvements will scale with your usage of large string literal unions.)
Improve the nonstrict-import lint's error message.
Fix bug in React.cloneElement for stateless functional components.
Add --max-warnings flag which allows Flow to exit with a non-zero exit code if warnings are present.
Add --file-watcher flag to ignore file system events.
Expose url.format's urlObj argument type.

0.65.0

Likely to cause new Flow errors:

New Features:

Improved inference of const bindings (lets and vars that are never reassigned), and introduced similar inference for function params. This allows Flow to be less pessimistic about refinements, especially inside closures. For example:
```
function f(val) {
  if (val == null) return () => 42;
  return () => val; // OK, since val cannot be null
}
```

Notable bug fixes:

Fixed regression in recheck performance

Misc:

implements now works on declare class in addition to class
declare module library definitions are now considered @flow strict
Fixed non-@flow strict import rule when in a cycle
Fixed incorrect spreading call properties and indexer properties from interface types
Fixed type-at-pos and coverage commands when they encounter Object.prototype and Function.prototype
Fixed a crash when hit with more than 1024 concurrent connections, by returning an error instead
Fixed the --timeout flag to work properly now that it connects instantly to the server monitor
Added an exit code (5) for when the server goes unused for a week and shuts itself down
Added a merge_timeout .flowconfig option to mirror the --merge-timeout CLI flag added in 0.64

Parser:

Fixed location of types containing trailing parens
Added implements to DeclareClass nodes

0.64.0

Likely to cause new Flow errors:

event is no longer recognized as a pre-declared global

Notable bug fixes:

An optimization intended to reduce redundant work during rechecks never fired after recent changes.
The implementation of React.ElementConfig involved a subtyping rule going the wrong way.

Performance improvements:

Typing info for files were retained in memory even after those files were deleted. This memory is now reclaimed.
Dependency calculations that occur during initialization and every recheck are now faster.

Misc:

Adds --merge-timeout server flag
Drops non-strict type args config
Fixes issue where flow-upgrade would not check .jsx files
Includes various fixes to docs

Parser:

Includes thin bindings to allow interfacing with the parser from C++. A prebuilt libflowparser.a is available for Mac and Linux on the GitHub release.
flow-parser-bin is an experimental node.js extension that provides access to the native (OCaml) parser from JS. flow-parser will be updated to delegate to flow-parser-bin if your platform supports it, and fall back on the slower compiled-to-JS implementation otherwise.
Trailing commas after a rest element in object patterns is now disallowed following the spec.

0.63.1

Bug fix:

Forgot to cherry-pick a diff to fix flow init, which was generated an invalid .flowconfig

0.63.0

Likely to cause new Flow errors:

Strict mode now treats function parameters as const
Declaring the exported type of a CommonJS module via declare var exports: T is deprecated in favor of declare module.exports: T

Notable bug fixes:

If a single file (or cycle of files) takes more than 100s to merge (which indicates something is horribly wrong), Flow will emit an error, stop merging that file/cycle, and continue merging the rest of the files.
Better handling of internal Flow errors during merge. A file (or cycle of files) with an internal error during merge will emit the error and set the type of the exports to any. This avoids cascading internal errors.

Misc:

flow get-def (used by IDEs for jump to definition) now behaves differently for variables introduced by an import or require(). Previously, it would show where the variable was created, in the import. Now it looks through the import and shows where the variable was exported.
The first steps in a large error message revamp are included in this version of Flow. Larger changes will follow in later versions.
Some small perf improvements
A bunch of libdef improvements. Thanks everyone for the PRs!

Parser:

Enforce that the rest property in object destructuring must be last.
Fixed a bug which banned methods named static in object types

0.62.0

Likely to cause new Flow errors:

Removed support for static properties on interfaces. Static properties were never well supported, and in most cases their types were not actually checked.
Fixed the polarity checker, which was not erroring in many places where it should have, like class statics.
Removed unsafe.enable_getters_and_setters option. Getters and setters are now enabled by default. Use the unsafe-getters-setters lint rule to disable.

New Features:

Improved error message locations and context around type errors in many cases
Added flow cycle subcommand, which prints a .dot describing the cycle for a given file.

Notable bug fixes:

Fixed bug where [lib] files outside the Flow root would not be watched for changes
Fixed a few race conditions, which would mask errors depending on the order of imports in a file.
Fixed refinements on opaque types with declared bounds.

Misc:

Added nonstrict-import lint rule, which requires that strict files can only depend on other strict files.
Added unsafe-getters-setters lint rule, which replaces the unsafe.enable_getters_and_setters flowconfig option.

Parser:

Added missing method property to object type property AST node.
Added support for properties named static for declare class and interfaces.
Changed to separate Variance AST node for +/- annotations, matching Babylon.
Improved performance by reducing GC pressure.

0.61.0

New Features:

Added an [untyped] section to .flowconfig, which treats all matching files as untyped, as if they did not have @flow. This differs from [ignore] in that [ignore] hides matching files from the module resolver /and/ the type system. This allows you to ignore types from third-party modules that are poorly-typed, typed for a different version of Flow, typed with a .flowconfig with different options, etc.
Experimental: Global find-references for class members.

Notable bug fixes:

Fixed an issue that caused IDE commands (autocomplete, get-def, etc.) to crash in certain cases.
Fixed an issue that caused IDE commands to return no results when the this type is nullable.
Fixed several bugs in lazy mode to avoid crashes and missed errors.

Misc:

Issue an error at every location where a particular non-existent module is imported, instead of just the first one in each file.
For the get-imports command, show every location where a module is imported instead of just one per file.
Quite a few libdef improvements.

Parser:

Disallow literals as prop keys in assignment destructuring.

0.60.1

Fixed a bug introduced in 0.60.0 in which Flow would not start if its temp directory did not exist.

0.60.0

Likely to cause new Flow errors:

These changes do not introduce new errors, but error suppressions may need to be moved to the new, more accurate locations.

Improved positioning of errors involving object types with incompatible indexers.
Improved positioning of errors involving the arguments of overloaded function types.

New Features:

Introduced a "server monitor" process that acts as an intermediary between client commands and the Flow server.

Previously, the server was only able to service one client request at a time and multiple connections would block until the server is free, preventing the server from telling the waiting clients why it's busy. Now, the monitor can accept many requests and respond more intelligently.

It is also able to detect when the server exits (e.g. when a package.json changes) and restart it transparently.
flow find-refs can now find local references to class properties.
New linters:
- unclear-type warns about uses of any, Object and Function, since they unsafely circumvent the type system.
- untyped-import warns when importing or requireing a module that does not have @flow.

Notable bug fixes:

Made the union created by $Values on a frozen object maintain singleton literals. That makes this pattern work:

const Enum = Object.freeze({
  X: 'x',
  Y: 'y',
});
type EnumT = $Values<typeof Enum>
('a': EnumT); // now errors. previously, EnumT was any string

Fixed Object.keys and $Keys on a dictionary of string literal keys, such that the result is now an array of the string literals instead of a generic string:
```
function f(dict: {['hi']: mixed}) {
  (Object.keys(dict): Array<'hi'>);
  (Object.keys(dict): Array<'bye'>); // error
}
```
Simplified the types mentioned in some error messages, like from "type application of polymorphic type: class type: Foo" to just "Foo"
Fixed get-def locations on class and object properties
Fixed get-def on refined object properties like the second prop in if (foo.prop) foo.prop(), which previously returned no results
Fixed non-termination bugs for predicates on classes, unions and polymorphic instanceof
Made recursion limit errors unsuppressable. Please report any such errors, they are always Flow bugs!

Misc:

Fixed compilation under ocaml 4.06
Added dependency on lwt from opam
Fixed error behavior of flow coverage in --quiet and --json modes
Made --json consistently imply --quiet in all commands, notably status
Fixed an issue where a new server may end up writing to the .log.old file
Various additions to lib definitions, thanks for your contributions!

Parser:

Implemented JSX spread children syntax
Made missing commas in export specifiers a parse error
Made import type * a parse error

0.59.0

New Features:

Adds a $ReadOnly<T> utility type which makes all properties on objects read-only. $ReadOnly<{ a: number, b: string }> behaves as if you wrote { +a: number, +b: string }. Read more about property variance on our blog.

type O = { a: number, b: string };

function fn(o: $ReadOnly<O>) {
  o.a = 42; // Error!
}

Allow read-only property initialization in constructors. Covariant properties are still read-only everywhere else in the class.

class X {
  +p: number;
  constructor(p: number) {
    this.p = p;
  }
}

25% performance improvement on full check time for large projects. (Results may vary.)
Enables lints in Try Flow. Enable lints with configuration comments like /* flowlint sketchy-null:error */.

Notable bug fixes:

Improves positioning for error messages involving exact objects. Notably, this bug caused error messages against React.Element to incorrectly point at library definitions instead of user code.

Misc:

Experimental implementation of find-all-references.
Enforces that exported class statics are annotated.
Improves typings for Node.js HTTP server listen() function.
Removes redundant information from some React error messages.

Parser:

Adds parser support for JSX fragments.
Various correctness changes to which identifiers error on reserved value and type names.
declare class with multiple extends is now a parse error.

0.58.0

Likely to cause new Flow errors:

Detect match failures with disjoint unions. Example:

type Foo = { type: 'A', ... } | { type: 'B', ... }`
function match(foo: Foo) {
  switch (foo.type) {
    case 'C': ... // dead branch, now error! (was allowed previously)
    ...
  }
}

New Features:

Support for user-defined "strict" mode. Using @flow strict instead of @flow in the header will now additionally fire lint rules that can be listed in the [strict] section of .flowconfig. Here are the current set of supported lint rules (more coming, contributions welcome!).

Error reporting:

Error location improvements for a bunch of common operations

Parser:

Tighten the AST around import statements, and rewrite their parsing logic
Improvements to locations of import statements, export statements, and declare statements

Perf:

Optimize a hot path in lazy mode, speeding up IDE commands
Optimize calculation of dependents, speeding up rechecks
Fix exponential blowup for large enums

Reliability:

Fix deadlock issues with flow ide
Gracefully handle crashes of the file watcher

Misc:

Don't list ignored files with flow ls
Remove flaky support for $Tainted types
Remove flaky support for --raw type printing
Various additions to lib definitions, thanks for your contributions!

0.57.3

Notable bug fixes:

Fixed a race condition which was causing the Flow server to hang during merge
Rebuilt the Windows binary

0.57.2

Misc:

Reverted the change which stopped call properties from flowing to object type dictionaries

0.57.1

Notable bug fixes:

Fixed a crash when a file goes from parsable to unparsable
Fixed a server crash when a client dies before receiving a response

Misc:

Added logging to show which components take a long time to merge

0.57.0

Likely to cause new Flow errors:

We've manually enumerated all the JSX intrinsics, so Flow might notice if you're misusing an intrinsic.
$Diff's implementation was rewritten. It should behave mostly the same, but will error on $Diff<ObjA, {x: string}> if ObjA doesn't have a property x

New Features:

Flow will now only check the files in node_modules/ which are direct or transitive dependencies of the non-node_modules code.

Notable bug fixes:

A handful of fixes for flow ide on Windows
Fixed a few bugs that would cause flow server to crash when flow ide exits
Fixed a regression in v0.56.0 which caused Flow to crash on import type * syntax
Fixed $ObjMap's behavior on optional properties
Various fixes for type destructors (like $PropertyType, $Diff, and $ObjMap)
Object type indexers no longer include call properties. So {[string]: boolean} is no longer a subtype of {(number): string}.
Fixed a bug where circular type imports would miss errors involving union types in rare cases.

Misc:

Updated Flow headers and license file from BSD3 to MIT
flow server will now write to a log file in addition to stderr
flow ls will now list the .flowconfig
flow ls will now list lib files, even if --all is not set
flow ls --imaginary will list nonexistent files. It's useful for speculating whether or not Flow would care about a file if it existed.
Added flow force-recheck --focus which tells a lazy server to start caring about certain files
Various small error message fixes
Lots of libdef updates! Thanks everyone for the contributions!

0.56.0

New Features:

Added a $Rest<A,B> type, which models the semantics of object rest
Added support for null prototypes, a la Object.create(null)
Added support __proto__ property in object literals and object type annotations

Notable bug fixes:

Improved support for React higher-order components, e.g. Relay fragment containers
Improved performance of flow focus-check for multiple files
Fixed type-at-post support for $ReadOnlyArray types
Fixed many cases where error messages were reported far away from the root cause.
Fixed find-refs for named exports

Misc:

Added experimental lazy mode for IDEs
Added <VERSION> token for suppress_comment option in .flowconfig
Removed support for $Abstract utility type
Removed support for flow typecheck-contents --graphml

0.55.0

Likely to cause new Flow errors:

Fixed a bug that caused unsoundness with $ObjMap.

New Features:

Flow is now capable of servicing some requests while it is rechecking. This should improve the IDE experience on large codebases.
Added $Call utility type.
Added $Compose and $ComposeReverse utility types.
Added support for spreading mixed into an object type.

Notable bug fixes:

Improve results from the find-refs command.
Allow null and undefined as React.createElement() config (fixes #4658).

Misc:

Miscellaneous code cleanup.
Located error messages related to functions at only the signature, rather than the entire range of the function body.
Improved error messages when this types are incompatible.
Properly check subtype relationships between callable objects.
Fixed a bug that caused mixed not to be properly printed from type-at-pos.
Improved error messages regarding incompatible Array type parameters.
Preserve some inference information across module boundaries.
Support assignments to shorthand method properties in object literals.

Typedefs:

Added captureStream() to HTMLCanvasElement and HTMLMediaElement.
Added HTMLOptGroupElement return type for document.createElement().
Added Recoverable and context to the repl module.
Fixed return type for AudioContext.createMediaStreamDestination().

Parser:

Various fixes to improve test262 compliance:
- Correctly disallowed various illegal constructs in destructuring patterns.
- Allow destructuring in catch.
- Added \u2028 and \u2029 to the list of line terminators.
- Allow unicode escape codes in identifiers.
Improved parse errors when using private properties outside of classes.
Disallowed reserved words as function param names in types (e.g. (switch: number) => void).

0.54.1

Notable bug fixes:

Fixed an issue where the server becomes temporarily unresponsive after a recheck and the client consumes all its retries.

0.54.0

Likely to cause new Flow errors:

Extending a polymorphic class must now explicitly specify the parent class's type args. That is, class A<T> {}; class B extends A {} is now an error, but class C extends A<string> {} or class D<T> extends A<T> {} is ok.
Improved accuracy of type checking calls of built-in methods (e.g. Object, Array, Promise)

Notable Changes:

Implemented private class fields, part of the Class Fields proposal.
Implemented "phantom" types (e.g. type T<Phantom> = any; type X = T<string>; type Y = T<number>, where X and Y are incompatible even though T doesn't use Phantom)
Unused suppression errors are now warnings instead
Improved errors involving polymorphic types, so that they now point to the source of the conflict, rather than the nested type args that are incompatible. This was a major source of errors in files other than where the problem was.
Improved errors involving structural subtyping, so that they now reference the objects that are incompatible in addition to the incompatible properties
Improved errors when an inexact object type flows into an exact type
Made rest parameters in object destructuring patterns sealed, and exact if possible
Improved definitions for some node fs functions
Improved performance by removing unnecessary caching

Misc:

Improved accuracy of type checking of React children in React.createClass
Fixed a bug related to instantiating a polymorphic type (e.g. type t = T<U>) with empty type args (e.g. var x: T<>)
Fixed polarity checking for property maps
Fixed a bug where polymorphic types were incorrectly checked for equality
Improved React definitions
Added a FLOW_TEMP_DIR env, equivalent to passing --temp-dir
Added a minimal libdef that defines the few things Flow can't run without, even when using no_flowlibs=true

Parser:

Added flow ast --strict to parse in strict mode without "use strict"
Added support for the RegExp dotAll ('s' flag) proposal
Added support for the private class fields proposal
Added support for destructuring defaults in assignments (e.g. given ({ x = 1 } = {}), x is 1)
Fixed issues related to let, yield, await, async, super and other reserved words
Fixed issues related to declarations in statement positions
Fixed issues related to destructuring patterns
Fixed issues related to IdentifierReferences, like shorthand object notation
Fixed issues related to class parsing, particularly new.target and async methods

0.53.1

Fixed a bug that sometimes crashed the server during recheck

0.53.0

This release includes major changes to Flow's model for React. The following links contain detailed documentation on the new model.

Defining components
Event handling
ref functions
Typing children
Higher-order components
Utility type reference

Please use the new flow-upgrade tool to upgrade your codebase to take advantage of these changes!

Likely to cause new Flow errors:

We are modifying how you define React class components. The React.Component class will now take two type arguments, Props and State (as opposed to the three type arguments including DefaultProps that React.Component took before). When your component has no state, you only need to pass in a single type argument. If your component has default props then add a static defaultProps property.
Flow used to not type React function refs at all, but now that we are typing refs code that used to just work may now have errors. One such error which can often be overlooked is that the instance React gives you in a function ref may sometimes be null.
Flow used to completely ignore the type of React children in many places. Intrinsic elements did not check the type of their children (like <div>), the type specified by components for React children would be ignored when you created React elements, and the React.Children API was typed as any.
In the past when typing children many developers would use an array type (Array) often with the React element type (Array<React.Element>). However, using arrays is problematic because React children are not always an array. To fix this, now use the new React.ChildrenArray type.

New Features:

Modeling advanced React patterns, like higher-order components, is difficult today because the types you would need are either not provided or undocumented. In this release we added a whole suite of utility types which are all documented on our website.

Notable bug fixes:

Flow used to have a bug where Flow would consider the following code as valid:

function MyComponent(props: {foo: number}) {
  // ...
}
<MyComponent foo={undefined} />; // this is now a Flow error

We now allow JSX class components to use exact object types as their props.
We now allow member expressions when creating JSX components, e.g. <TabBarIOS.Item>.
We now allow multiple spreads in a JSX element.
We have added a type argument to SyntheticEvents and correctly typed currentTarget using that type argument.

Parser:

We fixed miscellaneous character encoding issues.

Misc:

This release features a major re-architecture in how Flow typechecks modules against their dependencies. Earlier, Flow would do a "local" (per-module) typechecking pass followed by a global (cross-module) typechecking pass. Now, these passes have been merged. This change vastly improve Flow's memory usage on large codebases.
We found and fixed a couple of subtle bugs in the typechecking engine that caused stack overflows in some pathological cases.
We made various improvements to refinements. We now recognize vars that are only assigned to once as consts, so that we can preserve refinements on them through longer stretches of code. Some typeof cases have also been fixed.
We now support focus-checking multiple files. You can use it to debug issues easier and faster by telling Flow to focus on files of interest.
This release also includes lots of improvements to core type definitions. Thanks for your contributions!

0.52.0

New Features:

Flowlint - a linter built into Flow that you can configure to complain about things which aren't quite type errors.

Notable bug fixes:

Flow now enforces polarity on class supers (e.g. Flow will error on class B<+T> extends A<T> {} when A's type parameter is not covariant)

0.51.1

Changed linter (experimental, coming soon) to ignore lint errors in node_modules

0.51.0

New Features:

Added support for opaque type aliases
Added library definitions for the Node.js repl module (thanks @zacharygolba!)

Notable bug fixes:

Fixed library definitions for the Node.js cluster module (thanks @Corei13!)
Fixed the return type of EventEmitter#setMaxListeners (thanks @ahutchings!)
Added missing properties in the Node.js fs module (thanks @ahutchings and @rgbkrk!)
Fixed the length property on $ReadOnlyArray to be covariant (thanks @popham!)

Misc:

Improved error handling in our test runner
Fixed type error in our docs (thanks @stenehall!)
Fixed broken link in docs (thanks @vasyan!)
Fixed misleading typo in docs (thanks @joelochlann!)

Parser:

Fixed end locations of various statement nodes to include terminal rparen
Added separate DeclareTypeAlias and DeclareInterface AST nodes, matching Babel
Fixed locations of declared vars, classes, and functions in declare export stmts

0.50.0

Likely to cause new Flow errors:

Fixed a bug that suppressed unrelated errors when a missing annotation error was also suppressed.

New Features:

Added $Values type.

Notable bug fixes:

Fixed lints appearing in Try Flow.
Miscellaneous libdef improvements.
Fixed a couple bugs that could lead to missing push diagnostics when using the persistent connection.
Made $ReadOnlyArray covariant in variance checking.

0.49.1

Fixed an issue where flow init outputs a [lints] section (for experimental linting support, coming soon!) that includes all=false, which is already deprecated. Upcoming versions will support all=off instead.

0.49.0

Notable bug fixes:

Optimized performance when typechecking classes
Optimized performance of sentinel property checks involving large enums
Lots of libdef updates! Thanks everyone for the contributions!

Misc:

Fixed infinite recursion from array spread
Added experimental support for sending errors to an IDE over a persistent connection
Removed unused --libs flag on status, check, and start commands

Parser:

Fixed parsing scientific notation with decimal but no fractional part
Added support for parsing opaque types. Type system support coming soon.

0.48.0

New Features:

Experimental support for "lazy" typechecking. On large codebases, the Flow server will start up faster when using the --lazy flag with flow start or flow server, by only computing dependency information and not doing any typechecking. Instead, as files are touched, they are typechecked incrementally along with files that depend on them (and their dependencies). Relatedly, running flow focus-check on a file will only check that file and files that depend on it (and their dependencies).

Notable bug fixes:

Fixed crash in flow.org/try when using JSX (which was a regression in 0.46)

Misc:

Libdef updates! Thanks everyone for the contributions!
Removed the libelf dependency from Flow. This helps a few systems, like the standard node docker, that doesn't have libelf available.
Removed the strip_root .flowconfig option, since it doesn't make sense as a project-specific option and complicated IDE integration. Clients (including IDEs) can still use the --strip-root CLI flag as needed.
Improvements to handling of Object.prototype
Improvements to handling promotion of primitives to objects
Lots of refactoring to improve stability of commands and the typechecker

0.47.0

Likely to cause new Flow errors:

We are now sealing the object type that is passed to a React component's props. This means Flow will start complaining if you try and access unknown React props.
Strict function call arity checking is now on for everyone! The experimental.strict_call_arity option has been removed. For more, see https://flow.org/blog/2017/05/07/Strict-Function-Call-Arity/

Notable bug fixes:

There are a bunch of flow commands that will quickly try to check a single file, like autocomplete, get-def, check-contents, etc. We're trying to clean up what happens when these commands are given a file with @flow, @noflow, or no pragma. v0.47.0 contains a change intended to avoid doing extra work for files with @noflow or no pragma at all. v0.48.0 should continue to address this.

Misc:

Libdef updates! Thanks everyone for the contributions!
In a bunch of situations we're now propagating any types and continuing typechecking rather than stopping altogether.
Using a function as an object will now give that object a call property

Parser:

Optimized parsing of huge list of nested ternary expressions
The JS version of the parser now throws proper JS errors in the rare case that an OCaml exception is uncaught

0.46.0

Likely to cause new Flow errors:

We updated the type for the React.Component constructor, which means Flow can now infer the prop types based on the super(props) call in a react component constructor. This means components that weren't already declaring their prop types may start getting errors if there are any issues with their prop types.
We fixed the type of Promise.prototype.catch, which means code that uses .catch but wasn't properly handling the exceptional behavior might now have Flow errors

New Features:

We're updating how get-def (the jump to definition feature) works. It will now jump straight to the definition, even if it's in another file, rather than to the most recent assignment.
Starting in v0.47.0 we're going to complain when you call a function with more arguments than it expects. You can try it out in v0.46.0 with the .flowconfig option experimental.strict_call_arity=true. For more check out this blog post

Notable bug fixes:

Fixed a exponential blowup that could happen when using functions as callable objects
Fixed a bug where calling a function with a rest param wouldn't flow undefined to unfulfilled parameters.
Fixed a bug where declare class classes would be given a default constructor even if they extended another class

Misc:

Flow is faster! There are a bunch of perf wins in this release!
Lots of updates to library definitions! Thanks to all our contributors!
When using the Flow CLI, multiline errors will now show multiple lines of context
Replaced --no-suppressions with --include-suppressed which is a little bit more well-behaved.
Added --pretty flag to flow check for pretty-printed JSON

Parser:

Import expressions that appear in a statement list are now correctly parsed as import expressions (thanks @mikaelbr!)
Fixed the location of declare function declarations with predicates
Fixed the location of yield expressions with semicolons
Fixed the location of declare module declarations
Fixed a bug where array pattern with defaults like [a=1, b] = c was parsed like [a=(1, b)] = c.

0.45.0

New Features:

Flow now has proper unicode support in its parser!
Support for import expressions (Thanks deecewan!)
Introducing export type * from ..., an analogue of export * from ...

Notable bug fixes:

Fixed incremental checking bug when lib files are added or removed

Misc:

libdef and docs updates. Thanks for the PRs! Keep 'em coming!

Parser:

Unicode is finally supported! We've moved our lexer from ocamllex to sedlex, which supports unicode!
Fixed location of unary operator argument when surrounded by parens
Fixed a bug around using tagged templates expressions as call or member expressions

v0.44.1

Notable bug fixes:

Fixed the definition for HTMLBRElement, which extended itself :(

v0.44.0

New Features:

Another big perf win!

Notable bug fixes:

Internally, Flow wasn't always propagating the any type, which could suppress certain errors. We've fixed this, and Flow now notices some errors it was missing before

Misc:

Lots of libdef and docs PRs merged! Thanks to everyone for the help!
Minimum OCaml version is now 4.02
Partial support for object type spreads with generics

v0.43.1

Features:

Big perf improvement!

v0.43.0

Parser:

Fixed parser to disallow variance sigil for when using object spread.
Cleaned the flow-parser npm package of cruft. Thanks @zertosh!

Notable bug fixes:

Some internal refactoring helps Flow find some errors that it was previously missing.

Misc:

A ton of libdefs updates and documentation fixes. Thanks for all the PRs!

v0.42.0

Likely to cause new Flow errors:

New Features:

Object type spread (type TypeB = { ...TypeA };). You can now spread one object type into another! It is designed to mimic the behavior of spreading an object value in an object initializer, where only own properties are copied over.
Experimental new flow ide command, through which an IDE can establish a permanent connection with Flow. The IDE can send commands through this connection and Flow can push changes to the IDE.

Notable bug fixes:

Fixed a bug that would cause Flow's recheck to never terminate

Parser:

Support for object type spreads.

Misc:

Lots of updates to the builtin flow libs! Many thanks to all our contributors!
Object types with dictionaries are now considered sealed (you cannot add new properties to them). This is a minor change, since you can still write to the dictionary.
We've relaxed the restriction that React.PropTypes.oneOf must take an array of literals. You can now put non-literal values in the array.

v0.41.0

Notable bug fixes:

Lots of improvements to typing React components (which may uncover previously missed errors). This release primarily focused on these tweaks to React typing
Previously Flow did not track much about mixins from React.createClass components (including mixed in proptypes). This has been fix and may uncover new proptypes errors in createClass components
The internal React libdef for findDOMNode has been updated to include null in its return-type now (since this function can, indeed, return null)

Misc:

flow get-importers has been removed. It was rarely used and added an incredible amount of complexity to the codebase
Lots of updates to builtin flow libs

v0.40.0

Notable bug fixes:

Fixed an edge case when moving .flow files around in a haste project
Fixed bug which allowed you to add extra properties to exact objects
Fixed pretty printing of methods in type-at-pos and gen-flow-files
Fixed the @jsx pragma support's handling of trimming whitespace and filtering empty children. It now matches the behavior of the babel transform
Fixed some merge_strict_job exceptions
Fixed bug where Flow would ask for annotations in the object passed to React.createClass() even when they were not needed.
Fix so now you can use this and super in class method parameter defaults.
Optimization around union types

Parser:

Fixed a bug where line comments in certain positions were parsed as block comments

Misc:

Lots of updates to the builtin flow libs! Many thanks to all our contributors!
Some tweaks to error messages to make them easier to understand.
We are NOT removing weak mode in this release. See the discussion on #3316

v0.39.0

Likely to cause new Flow errors:

Previous usage of ReactElement<*> as a return type for render functions in React may now cause new errors now that a type propagation bug has been fixed. The general solution for this is to remove the return type for functions that return a React element (return types can be inferred by Flow, and Flow has never had a good way to express the type of a specific React element)
Several significant improvements to the locations Flow points to in error messages about objects and property accesses may move errors that were previously suppressed with a suppression comment to a new location

New Features:

You can now pass --no-suppressions to flow check in order to see what errors would show up in your codebase if you deleted all the error suppressions
New advanced debugging feature: You can now use the special $Flow$DebugPrint type to make Flow print a JSON representation of a type. Example usage: declare var flowDebug: $Flow$DebugPrint; flowDebug(someVariable);
Flow now supports constant-folding of tuple types. In general this means that Flow can now better understand rest elements and array-spreads when tuples are involved.
Flow now supports spreading types that are iterable (other than just Arrays)

Notable bug fixes:

Fixed several issues where Flow might not terminate while typechecking complex code patterns
Fixed an issue where a return type on a class constructor wouldn't properly define the type of objects the class generates if the type was too complex
Fixed an issue where Flow wasn't properly invalidating refinements after a yield expression
Fixed an issue where Function.prototype.bind() wasn't working properly on variables typed as "callable objects" in Flow
Fixed some issues where implements could cause a "Did not expect BoundT" error
Fixed an issue where Flow would previously give a cryptic error if the TERM environment variable wasn't set (Thanks @SamirTalwar!)

Parser:

Fixed an issue where the parser previously couldn't handle numbers greater than 2^32-1
The parser now errors on invalid variance + tokens in object types (like it already did for object literals)
Fixed an issue where the parser was incorrectly parsing JS directives (like "use strict")

Misc:

A few improvements to error messages relating to React PropTypes
Various core libdef updates

v0.38.0

Likely to cause new Flow errors:

There are some error-position improvements in this release. This means that if you use // $FlowFixMe (or any kind of suppress_comment), there's a chance the error may have moved out from under the suppression comment. You may see some unused error suppressions and some previously suppressed errors.
The behavior for tuples has changed to strictly enforce arity. This might cause errors depending on how you use tuples.

New Features:

implements - classes can now implement one or more interfaces and Flow will verify that the class properly implements the interfaces.
Local find references command: flow find-refs file line column - if there is a variable x at file:line:column, then this command returns the location of x's definition and the location of all reads and writes to x.
New shorthand for importing types in the same declaration that imports values: import {someValue, type someType, typeof someOtherValue} from 'foo'
Autocomplete now works with exact object types (thanks @vkurchatkin!)
Tuple types have been revamped. They now strictly enforce arity (docs). This will enable a bunch of future features around spreads and rest parameters. Stay tuned!
$ReadOnlyArray<T> is the common supertype of arrays and tuples. It's basically an array without stuff like push(), pop(), etc
A lot of tweaks to error messages to make them easier to understand.
You can declare getters/setters in declare class, interface's, object types, etc. They're still not safely checked, though.
Set emoji=true in the .flowconfig options and flow status will put emoji in the connection status messages (thanks @zertosh!)

Notable bug fixes:

You can now use template strings with require(), (e.g. require(`foo`))
Object.keys no longer returns methods since they are not enumerable
We've relaxed sentinel checks to allow you to check unknown properties. This is consistent with other conditionals
Flow recognizes type mismatches between the LHS and RHS of a sentinel check as always failing
Flow recognizes null and undefined checks as sentinel checks too
Flow used to assume that an array literal with a single element of type T had the type Array<T>. Now it's Array<*>, which lets you add elements of other types to the array.
Fixed $Shape to ignore "shadow properties", which are properties that don't really exist, but which have been referenced somewhere.
A few recheck bug fixes. The optimization from 0.37.0 wasn't properly rechecking when property variance changed, when certain locations changed, or when certain files are renamed.
Fixed mistake where computed getters and setters weren't erroring with unsafe.enable_getters_and_setters=false

Misc:

Lots of updates to the builtin flow libs! Many thanks to all our contributors!
make no longer complains if js_of_ocaml is not installed
flow check --profile now includes CPU time
flow server --profile and flow start --profile now print profiling info to the logs
Some optimizations for huge dependency cycles.
tests/ which run a flow server now save the .log file when they fail
Many tests now run with --no-flowlib, which make them much faster

Parser:

Parsing support for object spread properties
Fixed kind of a static method named constructor to be "method" rather than "constructor"
Support for new import type shorthand
OCaml AST for getters and setters is a little cleaner now
We now support defaults in setters
The bug fix for trailing commas in array lists was accidentally reverted and has been re-committed.

v0.37.4

Notable bug fixes:

1 more server recheck fix... Fourth time's a charm!

v0.37.3

Notable bug fixes:

1 more server recheck fix... Third time's a charm!

v0.37.2

Notable bug fixes:

Fix more issues in the server with rechecking changed files

v0.37.1

Notable bug fixes:

Fixed an issue in /try where Flow was using an ocaml regex API that couldn't compile to JS
Fixed an issue where a changed "literal" type in a module signature wouldn't cause the Flow server to recheck
Fixed an issue where an update of a module in a cycle may not properly recheck all of its dependencies in the Flow server

v0.37.0

Likely to cause new Flow errors:

There are some error-position improvements in this release, which means that if you use // $FlowFixMe (or any kind of suppress_comment), there's a chance the error may have moved out from under the suppression comment to a different location that's more indicative of the error position.

New Features:

LOTS of built-in libdef updates.
It's now possible to use import type/import typeof inside the body of a declare module to import types between libdefs.

Notable bug fixes:

Fixed an issue where dictionary types couldn't flow into a covariant-compatible version of themselves.
Fixed an issue where previously any + number would yield number. Now it correctly yields any.
Don't try to read from the filesystem at all when using flow check-contents where input is provided via stdin.

Misc:

The --old-output-format CLI flag is now gone.
Performance optimization that allows us to skip re-checking any recursive dependencies of a changed file when the file's types haven't changed
Improved error positions on property assignment type errors.
The parse() function in the flow-parser NPM module now takes either 1 or 2 arguments. Previously it would require both arguments in order to work.
Things typed as "callable objects" now inherit from Function.prototype rather than Object.prototype.

v0.36.0

Likely to cause new Flow errors:

We've been working on improving error messages, which often involves moving the errors closer to where the error is likely triggered. If you use error suppressing comments, some of your comments now appear unused, since the error moved.

New Features:

Lots of small improvements to error messages
flow ls --all will output libs and ignored files too
flow ls --explain will explain why Flow cares or doesn't care about a file
flow ls dirA/ dirB file.js will only list files under dirA/, files under dirB/, and file.js

Notable bug fixes:

Calling a method through computed properties (obj['method']()) is now supported
The client no longer consumes retries and waits patiently when the server is busy garbage collecting
Errors in lib files with --strip-root set now show context again

Misc:

Currently flow coverage and flow check-contents default to treating all input as if it has the @flow pragma. We will change this in a future version. To make this transition easier, both commands now have the flags --all and --respect-pragma, where --all is the current behavior and --respect-pragma is the future behavior.
Better error messages for unsupported destructuring.
Builtin libdef improvements
Fixed a TDZ issue which would allow let x = x;

Parser:

Fixed a bug where [...rest, 123] was incorrectly parsed

v0.35.0

Likely to cause new Flow errors:

Flow now knows that calling addEventListener() with 'click' and 'dblclick' will pass a MouseEvent to the listener. This means you might need to update foo.addEventListener('click', (e) => bar()) to foo.addEventListener('click', (e: MouseEvent) => bar()).

New Features:

Better error messages in a bunch of situations
flowtype.org/try now has an AST tab that shows the parsed AST of the example

Notable bug fixes:

Bindings that come from imports are now treated as const
Found and fixed a few situations where Flow was emitting redundant errors
Some if statement refinements were sticking around after the if statement in some situations when they should not have.

Misc:

Various libdef fixes and improvements
Various docs fixes and improvements
If foo has the typed mixed, we now allow foo.bar if you check that foo is not null or undefined.

Parser:

Better error message if you try to make a class property optional (currently unsupported)
Dropped support for let statements, which never made it into the spec (thanks @andreypopp)

v0.34.0

Likely to cause new Flow errors:

Dictionary types (i.e. {[key: string]: ValueType}) were previously covariant which proved to be a significant source of unsoundness. Dictionary types are now invariant by default in order to fall into consistency with other collection types. It is possible to opt in to explicit covariance with new syntax: {+[key: string]: ValueType}, but note that this is now enforced covariance -- which means the dictionary can no longer be written into (only read from). For mutable collections, consider using Map/Set/etc. Please see this blog post for more information on variance.
Object property types are now invariant by default. New syntax allows explicit opt-in to enforced covariance: type T = {+covariantProp: string}. Please see this blog post for more information on variance.
Object method types are now covariant by default. So this: type T = {covariantMethod(): string} is the same as type T = {+covariantMethod: () => string}. Please see this blog post for more information on variance.

New features:

New empty type annotation. This is the "bottom type" which is the type that has no possible values. This is mostly useful for asserting impossible types right now (see the commit description for more details).
All server commands now have a --quiet flag to suppress server-status information that would otherwise be printed to stderr.
It's now possible to specify an "@jsx" pragma to override the implicit default of React.createElement. See the commit message for more details.
async iteration is now Stage 3, so Flow now supports async generators and for-await-of statements.

Notable bug fixes:

Calling get-def and autocomplete on specifiers in import statements now works properly and links in to where the specifier is exported in the other file.
Generator.prototype.return now returns a possibly-unfinished IteratorResult object rather than a definitely-done iterator result. See #2589 for more details.
Fixed an issue with inferring the proper return-type of iterators coming from a generator with multiple return-types. See #2475 for more details.
Fixed an issue where a non-polymorphic class-instance used as a CommonJS export wasn't omitting underscore-prefixed members when the munge_underscores config option is set to true.
Fixed an issue where Flow would previously not consider non-@flow files when re-calculating type dependencies on a change to the filesystem. This caused sporadic issues where Flow might think that a module is missing that actually is not! This is now fixed.

Misc:

Significant memory usage optimizations by normalizing common aspects of the many "reason" structures stored in memory to use ocaml variants.
Significant memory usage optimization by compressing the contents of the shared heap used by the persistent server.
Parser now allows for duplicate properties and accessors in objects per the latest ES spec.
Flow parser is now tested against esprima3 tests
yield expressions no longer evaluate to an optional type. This is unsound, but the inconvenience was so prevalent that we decided to relax the issue for now. See #2080 for more details.
Various core libdef updates.

Parser breaking changes:

Updated the parser to use the latest ExportNamedDeclaration and ExportDefaultDeclaration nodes from ESTree rather than the outdated ExportDeclaration node.
export default class {} now correctly emits a ClassDeclaration rather than ClassExpression per this estree issue.
Updated ExportSpecifier property names from id -> local and name -> exported per the latest ESTree spec.
Update ExportBatchSpecifier to a ExportNamespaceSpecifier node per the latest ESTree spec.
Renamed SpreadElementPattern -> RestElement per the latest ESTree spec.
Node-properties of ObjectPattern are now named Property and RestProperty per the latest ESTree spec.
Use a Super node now instead of an Identifier node per the latest ESTree spec.
{ImportNamedSpecifier and ImportDefaultSpecifier nodes now use proper local and remote property names, per the latest ESTree spec.
The mixed type annotation is now represented with a special MixedTypeAnnotation node (same as Babylon has been for a while).
The NullTypeAnnotation annotation node is now called NullLiteralTypeAnnotation in order to match Babylon.

v0.33.0

Likely to cause new Flow errors:

Its now an error to add mixed to number

New features:

suppress_comment now defaults to matching // $FlowFixMe if there are no suppress_comments listed in a .flowconfig
The Flow server no longer restarts itself when a package.json file is changed
The Flow server no longer restarts itself if a libdef file is touched, but not actually changed
Added support for using Iterables with Promise.all() (thanks @vkurchatkin!)

Notable bug fixes:

Fixed an issue where some predicates could cause Flow to crash
Fixed an issue where we weren't properly looking things up on Function.prototype and Object.prototype
Fixed an issue where Flow could crash when extracting coverage on empty types
Fixed an issue where long paths that are ignored could give a bunch of warnings on Windows
Fixed an issue where flow get-def wouldn't hop to the location of a type coming through an import type
Fixed an issue with dictionary types where using an any-typed variable as a computed-property lookup results in the wrong property-value type
Fixed some issues where Flow wouldn't allow definition of properties or methods called "static" on classes
Fixed an issue where Flow wouldn't permit throws at the toplevel of a module
Fixed an issue where adding a file to [libs] with an extension not listed in module.file_exts, it would previously be silently ignored
Fixed an issue where import * as on a declare module.exports: any; libdef would not result in a module with every possible named export
Fixed a parsing issue where "JSX attributes must only be assigned a non-empty expression" syntax errors sometimes point to the wrong line
Fixed parsing of setter methods with destructured parameters
Fixed a parsing issue where it wasn't possible to use get or set in object short notation
Fixed a parsing issue where we previously weren't allowing strings in async method names: x = { async 123() { await y; } }
Fixed an issue where the parser previously wouldn't recognize the u regex flag

Misc:

Various built-in libdef improvements
Various significant shared memory optimizations
When a package.json key is read by Flow during module resolution, don't wrap it in quotes
Added parser (but not yet typechecker) support for new.target
Added --pretty flag to all commands that have a --json flag
Flow now prints an exception instead of segfaulting if there is a heap overflow
Only print JSON for flow coverage when --json is passed (thanks @aackerman!)

Parser breaking changes:

Removed 'lexical' property from SwitchStatement
Removed guardedHandlers from TryStatement
Use AssignmentPattern for function param defaults to match ESTree
Use RestElement for function rest params to match ESTree
Fixed the location info for ExpressionStatement
Fixed the location info for CallExpression and MemberExpression

v0.32.1

Notable bug fixes:

If Flow runs out of heap space, it now throws an exception instead of segfaulting.

v0.32.0

Likely to cause new Flow errors:

If you check that an object obj has a property foo that Flow doesn't know about, Flow will now refine obj.foo to the type mixed. If you then try to use obj.foo in an unsafe way (like as a function), you might start seeing errors mentioning property `foo` of unknown type. The fix is to either fix the type of obj to include an optional property foo, or to rewrite code like obj.foo && obj.foo(x) to typeof obj.foo === "function" && obj.foo(x)
We've fixed the type of Array.prototype.find to reflect the fact that it can return undefined
We've tightened up the checking of tuple types

New Features:

New syntax for exact object types: use {| and |} instead of { and }. Where {x: string} contains at least the property x, {| x: string |} contains ONLY the property x.
Flow now watches for .css, .jpg, .png, .gif, .eot, .svg, .ttf, .woff, .woff2, .mp4 and .webm files (this list is not configurable at the moment). We call them resource files. If you require a resource file, you get a string (except for .css files, for which you get an Object). You should still be able to use module.name_mapper to map a resource file to a mock, if you prefer.
We're starting work on flow gen-flow-files, which consumes Flow code and outputs .flow files containing only the types exported. It's alpha-level and we're still iterating on it, so use at your own peril!

Notable bug fixes:

Fixed a bug where, if a module has a CommonJS export of type any, then the types it exported were also given type any
v0.31.0 contained a regression where arrays and promises required more annotations than they should. This is fixed.
Fixed use of tagged unions with intersections and generics.
Jump to definition for overridden methods was jumping to the wrong method. This is now fixed.
flow coverage had a non-termination bug. This is now fixed.

Misc:

Lots of improvements to the builtin libdefs! Thanks for all the pull requests!

v0.31.0

Likely to cause new Flow errors:

Fixed an issue where an any type could actually prevent errors from showing up in places that actually should surface errors
Array.isArray() now refines arrays to Array<mixed> instead of Array<any>

New Features:

When the munge_underscores option is set, Flow no longer requires annotations on exported classes with munged methods. (thanks @pvolok!)
Added a new "magic" type called $PropertyType<T, 'x'>. This utility extracts the type of the property 'x' off of the type T.
It is now possible to use leading-| and & on any type annotation, not just in type aliases

Notable bug fixes:

Significant perf improvements on checking disjoint unions
Fixed an issue where flow start would sometimes hang on Windows
Fixed an issue where flow get-def on a react element would always point to the internal react libdef (rather than the component that defines the element)
Error messages for builtin types are now more descriptive in more scenarios
Fixed the order in which destructured function params were processed. This previously caused issues with variable references within the destructuring
Error messages for union type errors now point to more helpful locations in some cases
Fixed a long-standing confusing error message blaming property accesses on the "global object" (when the global object isn't actually involved at all)
Improved error message when trying to import a named export from a module that only has a default export
Fixed an issue where Promise.resolve(undefined) would not always error as it should
Fixed an issue where async functions that return void do not properly enforce their return type
Fixed an issue where aliased object types may not error when an invalid property is accessed on them
Fix flow coverage when reporting coverage on empty files
Printed types in some circumstances (autocomplete, type-at-pos, etc) are now capped on size to prevent overflows in tools that consume them. When the size overflows the cap, ... will be printed as an overflow placeholder
Various built-in libdef updates

v0.30.0

Likely to cause new Flow errors:

Fixed React.PureComponent's definition, so previously missed errors are now reported
The definition of console in the build-in libdef has been filled out and is no longer any.

New Features:

From now on we're going to start publishing Windows builds with each release. Please report any issues you have!
Forward references in type annotations: you can now reference a class in a type annotation before the class is declared
T is now covariant in Class<T>. So if class B extends class A, then Class<B> is now a subtype of Class<A>
Flow now lets you destructure objects with computed keys.
flow check-contents --respect-pragma - check-contents checks whatever you give it, regardless of @flow or @noflow. This option changes the behavior to respect the pragma.
flow check-contents will now report parsing errors too (thanks @nmote!)

Notable bug fixes:

Fixed --trace behavior when we unify types
Fixed situation where the client could spin-wait for the server
Fixed non-termination triggered by certain calls to Function.prototype.{apply,call}
Fixed issue where "Duplicate module provider" errors would stick around even after being fixed.
(Windows) Fixed how the flow client tails the flow server's logs
(Windows) Fixed the log rotating
(Windows) Fixed issues where Flow would report Process Handles instead of Process IDs, leading to bad messages and a non-functional flow stop
(Windows) Fixed build outside of a git or hg repository
(Windows) Better error messages when you have paths that are too long

Misc:

undefined is now just a global variable declared in the libdefs
Various built-in libdef improvements
Nifty PR from @nmn which teaches flow that nullableArr.filter(Boolean) is non-nullable

v0.29.0

New features:

Tagged unions of interfaces should now work in the same way that tagged unions of type aliases work.

Notable bug fixes:

Building Flow outside of a git or hg repo should work now
If you declare an overloaded function with one overload that handles a string argument and one overload that handles a number overload, then passing in a union string | number should now work.
Annotations for destructured patterns now work like non-destructured patterns. Types no longer flow through these annotations, rather they are checked against the annotations and then the annotations are used instead.
Allow import {type} from "Foo"
We had a bug where Flow wouldn't complain about duplicate haste modules. This is now fixed
Fix for edge case hit when using experimental.strict_type_args=false and type parameter upper bounds.

Misc:

Various built-in libdef improvements
Some error location fixes
Lots of Windows support work. Should build and mostly work, but not fully stable yet.
Some performance wins
flow check --json --profile now includes profiling info in the JSON response

v0.28.0

Likely to cause new Flow errors:

Significant fix to matching members of union types. This fix very likely surfaces typechecking issues that were missed before. Check out the blog post for more details
Variables in template literals are now subject to the same rules as variables being concatenated with a string using +. This may surface template strings with variables that may not safely coerce to a string.
Type arguments are now required to be specified for type annotations that have type arguments. Previously they were implicitly filled in as any -- but now this must be explicit. This behavior is toggleable via experimental.strict_type_args=false, but this config option will be removed in a near-term future release. There is also a codemod to help you automatically update old code.

New Features:

Support for declare export within declare module bodies. When declare export is used, the module being declared is inferred as an ES module. This enables the ability to export both named and default exports using declare module.

Notable bug fixes:

Fixed a bug on OSX where Flow might crash in rare circumstances
Fixed an issue where automatic semicolon insertion for class properties wasn't behaving properly in some circumstances.
It is no longer required to annotate a property on an exported class if an initializer value is present. Instead, the initializer value itself can be directly annotated. This makes annotation of class properties that are functions much less cumbersome.
Fixed a race-condition where Flow may not properly error when it encounters two modules with the same name (using the "haste" module system)

Misc:

Flow now looks for @flow in all comments at the top of the file (rather than just the first). It's also possible to configure this using a new max_header_tokens .flowconfig option.
Various built-in libdef improvements
Performance improvements for the flow coverage command

v0.27.0

Notable bug fixes:

Fixed a filesystem race condition where Flow would note that a directory exists just before it is deleted, then try to read the directory
Fixed some issues with disjoint unions that have complex definitions
Fixed an issue where functions that come after a return aren't analyzed (even though they are hoisted)
A few updates to the Node.js library definitions
Fixed a race condition when adding specific properties to dictionary types
Various performance improvements
--strip-root is now applied to the output of --verbose
Fixed an issue where duplicate method declarations weren't understood correctly

v0.26.0

(@gabelevi mistakenly listed a few v0.26.0 changes as being in v0.25.0. The Changelog has been updated to reflect reality. Sorry!)

Likely to cause new Flow errors:

Flow now understands stateless functional React components, which may reveal many errors if you use them heavily.

New Features:

Support for stateless functional React components!
Support for the exponentiation operator (**)
New flow ls command to list the files that Flow can see.

Notable bug fixes:

Fixed parsing of /x/* 5 */y/ - that's no comment!
Fixed parsing of newlines or comments at the end of a template literal.
Fixed an incremental bug where Flow didn't notice new JSON files in certain situations
Fixed a parsing service crash when a file appears and disappears suddenly.

Misc:

Restored a bunch of deprecated/experimental/browser-specific APIs to the builtin flowlib and made them optional
Fixed up parser tests and further integrated them into CI.
Lots of refactoring!

v0.25.0

Likely to cause new Flow errors:

@marudor made a tremendous effort to clean up the builtin flowlib definitions, adding missing things, fixing annotations, and removing non-standard and deprecated features. If you're relying on these things, then you may have new errors.
In the past, generic types could leave off the type arguments. Flow is moving towards making these required. Applying type arguments to a polymorphic type (e.g. Map<string, number>) is like calling a function. If writing my_function was the same thing as writing my_function(), it would be really difficult to pass functions as values. Similarly, by making type arguments required, it frees us up to do more with our polymorphic types. If you have a polymorphic type with default types, like type Foo<T = number>, you can now write Foo<> to apply 0 type arguments to the polymorphic type Foo.

To ease migration, v0.25.0 still allows you to omit type arguments by default. v0.27.0 will start enforcing that type arguments are always supplied. To enable this behavior in v0.25.0, you can add experimental.strict_type_args=true to the .flowconfig.

New Features:

declare module.exports: type; <-- this is the new syntax to declare the CommonJS export type. Previously, you had to write declare var exports: type;. This can be used in .flow files and in declare module declarations.

Misc:

Now Flow builds on OCaml 4.03.0
Fixed up the parser tests (thanks for the help @marudor!) and have started running those in CI
A ton of refactoring and clean up

v0.24.2

Fixed a bug where Flow might run out of memory in a repository with a lot of non-flow files

v0.24.1

Fixed a bug where autocomplete can show internal variable names for files that use destructuring

v0.24.0

New features:

Many common errors now have more contextual error messages. Check out the test file changes to see what this looks like!
If a <PROJECT_ROOT>/flow-typed/ directory exists, Flow will now assume it is a [libs] directory by default to help reduce the amount of out-of-the-box configuration that is necessary. (Thanks @splodingsocks!)
Support for specifying a default on a type parameter declaration. For example: type Iterator<Yield, Return=void, Next=void> = ....
- NOTE: The pull request to add syntax support for this is pending, so it may be necessary to wait on that to ship before using this feature

Notable bug fixes:

Fixed the flow coverage command.
Fixed crashes when running type-at-pos and dump-types over destructuring.
Fixed a refinement bug when evaluating a method call on a refined variable in some cases.
Fixed an issue where some system commands could throw if Flow attempts to read a directory that it doesn't have permission to read.
Fixed a bug where the inferred return type of some functions that might return void could end up as exactly void (rather than a union of the possible return types).
Fixed a bug where the server would crash if a .json file is deleted after a server was already running.
Type applications that don't have the right number of type parameters (i.e. Map<number>) now emit a more clear error.
Lots of dom/bom improvements sent in from contributors! (big thanks to @marudor for lots of these!)

Misc:

Better locations for logical operator errors
Better error message sorting:
- Sorts errors in .json files by name coalesced with .js files
- Sorts all internal errors before parse errors before type/inference errors
- Sorts lib file errors before source file errors
Perf improvements for some comparisons of polymorphic types

v0.23.1

Fixed parsing of JSON files with duplicate object keys

v0.23.0

Likely to cause new Flow errors:

When you refine a mixed variable with typeof myVar === 'object', we used to refine the type of myVar to null | Object. Now it is refined to null | {[key: string]: mixed}. This means myVar.prop has the type mixed rather than any.
Removed non-standard Promise methods. Previous versions of Flow specified the type of Promise.prototype.done and Promise.cast, which are not standard or implemented in browsers. If you rely on a polyfill that does provide these methods, you can redeclare the Promise class in your project's local libs folder. Note that you need to copy the entire class declaration from lib/core.js in order to add methods and properties. It's not currently possible to extend the builtin declarations, but it is possible to redefine them.

New features:

Errors involving union or intersection types now include more information about why the various branches failed
flow init now has more command line flags to specify what should be in the created .flowconfig
flow ast now can parse JSON files
Comments are now supported in .flowconfig files. Lines starting with # or ; are ignored
In the [ignore] section of a .flowconfig you can now ignore a relative path with <PROJECT_ROOT>/path/to/ignored
Most flow commands have an --ignore-version flag to skip the version check specified in the .flowconfig.
Added a module.use_strict option to the .flowconfig. When it is true, Flow will treat every file as if it has the "use strict"; directive.
Using strict equality, you can now refine the number type into number literal types. (e.g. after x === 0 Flow now knows that x has the type 0)
Flow no longer requires return type annotations for exported functions if Flow can fully infer the type.
flow force-recheck FILE1 FILE2 command tells the flow server that FILE1 and FILE2 have changed and that it should recheck. This is intended for tooling that might be racing the file system notifications.
Flow is smarter about what !x evaluates to for various types of x.
<Foo /> is now allowed when Foo is a string. If $JSXIntrinsics is defined, Foo must be a subtype of $Keys<$JSXIntrinsics>
Support for class decorators in addition to property decorators (also gated behind the esproposal.decorators config option). Thanks @marudor!

Notable bug fixes:

Disallow (obj: SomeClass) except for when obj instanceof SomeClass
Fixed setting temp dir via the .flowconfig
Added missing all flag to the .flowconfig
Fixed a bug when destructuring a non-literal object type using a pattern with defaults
Fixed the --strip-root flag for displaying errors
Classes can now have properties named async
Fixed refinements like if (foo[0]) { ... }, which should work like if (foo["prop"]) { ... } does. That is, Flow should remember that foo[0] exists and is truthy.
Fixed parsing docblocks with CRLF line endings
Fixed autocomplete within if statements

Misc:

Added more info and structure to JSON output, without removing or existing fields
Loads of improvements to the builtin libraries
Bunch of perf improvements
Clarified some errors messages and error locations
flow start --json will start a server and output a JSON blob with info about the new server
Slowly improving tracking side effects in switch statements
Some improvements to pretty printing errors
Object.values() and Object.entries return Array<mixed> and Array<[string, mixed]> respectively, since Flow currently is never sure that it knows about every property in an object.

v0.22.1

Patch release to fix some JSON parsing issues that went out in v0.22.0

v0.22.0

Likely to cause new Flow errors:

Overhaul of Flow's understanding of React APIs. Some of these updates remove old/deprecated React APIs. Check out the new React docs for an overview of how things work.

New features:

Flow now gives precedence to library definitions over non-@flow implementation files. This means that it should no longer be necessary to specify a node_modules dependency in the [ignore] section of your .flowconfig if you have a library definition defined for that dependency.
Significant improvements to Promise.all: We now preserve the type of each item in the array passed to Promise.all() so that it may be propagated through to the resulting .then() handler.
We no longer try to parse files that are not marked with an @flow pragma. We anticipate this will improve performance for projects with large, non-Flow node_modules directories.
Classes with static members are now subtype-compatible with structural object types
It is now possible to specify a leading | or & for type aliases of long unions/intersections. This is useful, as one example, for disjoint unions with a large number of members (where each member sits on a new line):

type MyDisjointUnion =
  | {type: 'TypeOne', ...}
  | {type: 'TypeTwo', ...}
  | {type: 'TypeThree', ...}
  ...
;

Bug fixes:

Fixed an issue where an intersection of two object types did not always properly combine to match objects with members on both sides (#1327)
Fixed an issue where an object of some intersection type could not be used with the spread operator (#1329)
Fixed an issue where refinement-testing on an object of an intersection type wouldn't always work (#1366)
Fixed an issue where an intersection of function types with a common return type should type check against function types with union of param types
Fixed an issue where refining obj['abc'] didn't behave quite the same as refining obj.abc
Fixed an issue where usage of flow get-def on the left-hand side of a require() wouldn't hop through the require() and to the actual location of the definition
Fixed an issue where Flow would not give a clear error when trying to use import type to get a non-type export
Fixed an issue flow dump-types --json was not as robust as it could be against outputting valid JSON in the event of certain kinds of errors
Fixed an issue where Flow would not give a parse error if multiple ES exports with the same name are exported from a single ES module
declare class declarations now properly define a built-in name property (like real class declarations do)

v0.21.0

Likely to cause new Flow errors:

ES6 react classes without state should now extends React.Component<DefaultProps, Props, void> (previously it was extends React.Component<DefaultProps, Props, {}>)
ES6 react classes with state should declare state: State;
Before, it was possible to test for properties in objects which did not exist (if (o.noSuchP === foo) { ... }). These kinds of checks will now cause a type error.

New features:

Autocomplete for jsx properties
Typed JSX intrinsics. This means you can list which jsx intrinsics exist (like div, span, etc) and specify which properties they have.
Syntax for declaring variance at definition. For example, interface Generator<+Yield,+Return,-Next> {...}. Still pending transpiler support though.
Refining string and union types with string equality now properly refines the types.
Support for export * as from @leebyron's Stage1 proposal. Babel support here

Notable bug fixes:

Fixed bug with class expressions due to this type
Fixed autocomplete for this
Recognizes exhaustiveness in switch statements with default case.
Fixed "Did not expect BoundT" errors
Fixed infinite loop in certain recursive types
Fixed an incremental mode issue with deleted files
Fixed an incorrect refinement when assigning an object to a variable.

Misc:

Some internal errors now will be made user visible instead of silently failing. They generally mean that Flow has some bug or is making an untrue assumption/assertion. If you see these please report them!
Improvements to how we report certain types (type application, optional types) via our APIs
Various sentinel improvements, including boolean sentinels
Various improvements to the builtin flow libraries (thanks everyone for the pull requests!)

v0.20.1

Bug fixes:

Ironed out some issues with the this type

Misc:

find package.json using normal parsing phase

v0.20.0

New features:

Initial support for a this return type for class methods
Big improvements on error messages for unions and intersections
import typeof * as now allows for concise access to the type of the ModuleNamespace object
Flow now understands how to require()/import a .json file

Bug fixes:

Fixed an issue where nested unions and intersections might not typecheck as expected
Fixed issue where declare type wouldn't work in a declare module that has an exports entry
Fixed an issue where the error formatter could fatal in some rare cases
Fixed a bug where Function.prototype.bind would lose the types of the params on the function it output
Fixed some variance bugs in the built-in Array.prototype library definitions
Fixed a bug where using a list that doesn't contain ".js" in module.file_ext would cause internal flow libs to be ignored
Fixed autocomplete to work better with general Function types
Fixed some issues with const refinement
Fixed various issues with export * from (it should work fully now)
Fixed a bug where Flow might crash when an export has a wildcard typeparam

Misc:

Some improvements to DOM and Node libdefs
Various error position relevancy improvements
Significantly improved general understanding of special functions like Function.prototype.{bind,call,apply}
Improved error messages for import statements where the remote exports don't exist (or may be typo'd)
Improvements to understanding of deferred initialization of let variables
flow get-def will now hop through lvalues in variable assignments for more fine-grained "hop-tracing" of a variable back to its definition
Objects with a callable signature can now be passed in to type positions that expect a function with a matching signature
Significant improvements to efficiency/perf when recalculating types based on a change to a file with an already-running Flow server

v0.19.0

Likely to cause new Flow errors:

Flow syntax is now disallowed in non-@flow files. Use @noflow to work around this
import type * as Foo is now disallowed in favor of import type Foo
require() can only take a string literal
ES6 react classes without defaultProps should now extends React.Component<void, Props, State> (previously it was extends React.Component<{}, Props, State>)
ES6 react classes with defaultProps should declare static defaultProps: DefaultProps;
Flow notices errors it missed before in React.createClass() react components
Flow is now stricter about using uninitialized variables
Stricter checking of in keyword

New Features:

flow coverage command
null type annotation
Support for class field initializers, gated by .flowconfig options
You can now override flowlib definitions with local lib files
Basic support for computed properties
Declaration files (.flow files). Long story short, if foo.js and foo.js.flow exist, Flow will prefer the latter and ignore the former.
declare export - a way to declare the exported types in a non-lib file
Array rest destructuring assignment support

Notable Bug Fixes:

Fix "package not found" error in some symlink situations
Object indexer should not imply callable signature
Default param values can reference earlier params
Fixed a case where we weren't substituting type parameters properly
Fixed a situation where Flow would prefer an unchecked module over a library definition

Misc:

Add --root arg to most client commands
More repositioning of error locations
flow get-def: Jump to module named in import statement
Lots of fixes to make flow commands smarter about connecting to the server
Smarter refinement in a bunch of situations
freeze imports on all modules, and require() on ES6 modules
You can now spread classes, like var {x} = new Foo()
Interfaces now can be callable
If you've refined an object property, that refinement survives access through a destructured refinement
Better autocomplete results for primitives, objects, functions and unions
flow server will write to log file in addition to stdout/stderr

v0.18.1

Likely to cause new Flow errors:

Flow is now stricter (and more consistent) about how null works when used as an initialization value for object properties that are mutated later. So let o = {prop: null}; o.prop = 42; is now an error and requires that null be annotated: let o = {prop: (null: ?number)};.
The return type of RegExp.prototype.match() is now properly annotated to return a nullable.

New Features:

We now take advantage of the guarantee that const variables are read-only when doing refinements. This means that refinements of const variables now have much fewer caveats than refinements of lets or vars. For instance, it's safe to depend on the refined type of a const within a local function created in scope of the refinement, even if the function escapes.
We now track which external variables a function actually modifies, and forget refinements to only those variables when a function gets called.
New config options: esproposal.class_static_fields=warn|ignore and esproposal.class_instance_fields=warn|ignore. This allows the new ES class fields syntax to be ignored.
New config option: module.system.node.resolve_dirname. This allows configuration of the name of the node_modules directory (or directories) used by the node module system. This is similar in behavior to webpack's resolve.moduleDirectories config option.
Added support for a <PROJECT_ROOT> token in the template string for the module.name_mapper config option. This token will be replaced with the absolute path to the current project root before mapping the module name. For example: module.name_mapper='^$.*$$' -> '<PROJECT_ROOT>/src/\1' would canonicalize an import from "foo/bar/baz" to "/path/to/root/of/project/src/foo/bar/baz".

Notable Bug Fixes:

Fixed a bug where we were enforcing TDZ on reads to let/const, but not on writes.
Fixed a bug where any refinement clearing that occurred as a result of a function called in an if-statement was forgotten, meaning that lots of refinements were incorrectly allowed to stand.
Fixed a bug where generator code couldn't previously call yield without an argument.
Several improvements to generators.
Various improvements and bug fixes for TDZ enforcement.
Various other small improvements to refinements.
Fixed an issue where errors could sometimes spew to stdout rather than stderr.

Misc:

Removed flow single as it is effectively redundant with flow check --max-workers=1.
The "server starting" output now gives more insight into progress.
flow --version is now deprecated in favor of flow version.

v0.17.0

New Features:

New default error message format shows the code inline (thanks @frantic!)
Autocomplete will keep trying even if the file fails to parse
You can configure which file extensions Flow looks for (thanks @eyyub!)
Support for negative number literal types.

Notable Bug Fixes:

Incremental make on OSX now works when lib/ files change
Fixed some issues around const destructuring
Fixed some issues around destructuring in for-of and for-in loops
Some emacs plugin fixes
Object spreads are now handled in the correct order
Generator return() and throw() methods are now supported
Object types now allow keywords as the object keys (thanks @samwgoldman!)
importing & exporting default using named specifiers is now supported
Flow now understands this in class static methods and this.constructor in class instance methods (thanks @popham!)
Fixed bug with array spreads
Understand that all classes have a static name property

Misc:

Improved flow find-module
More error location improvements
Object can now be called as a function to cast things to objects
We've tried to standardize the error codes with which Flow exits. Some exit codes have changed, but the ones you probably use should be the same. At the moment they're only documented in the code
Flow understands the value of a negated number literal

v0.16.0

Likely to cause new Flow errors:

Some module exports that didn't require annotations before may now require annotations

New Features:

Let/const support! Finally! (Huge props to @samwgoldman...again :] )
Support for mixins on declare class library definitions

Notable Bug Fixes:

Improvements to types inferred from switch cases that fall through to a default cause
Further improvements on symlink support

Misc:

Significant performance improvements (both for initial start-up time and running re-calculation time)
Improved --traces output

v0.15.0

Likely to cause new Flow errors:

Flow now treats class identifiers as being let bound. You cannot refer to a class before it is defined.
If you accidentally use a tuple type instead of an array type for a rest param then Flow will complain
You cannot use this before super() in a derived constructor, per ES6 semantics
Our dictionary property (aka indexer property) support is much more robust and catches things it previously missed.
We weren't properly enforcing optional properties in interfaces and declare class. Now we are.

New Features:

Generators support, courtesy of @samwgoldman
The number of worker processors is now configurable, defaults to the # of CPUs
If Flow knows the value of a boolean expression, then it will know the value of that expression negated.
Flow can remember refinements for things like if(x.y[a.b])
export type {type1, type2} syntax

Notable Bug Fixes:

Fixed issue where Flow would still read package.json for [ignore]'d path
Fixed some leaky annotations that let data flow through them
Fixed instance and class types to be considered truthy
Flow still initializing message now fits in 80 chars, compliments of @spicyj
No longer will report hoisted declarations as unreachable code
Fixed issue with how Flow chooses its tmp dir
An async function can return a T or a Promise<T> and it means the same thing
Fixed Flow forgetting about optional properties after an assignment refinement
Fixed parser issue around future reserved keywords
Optional parameters and rest parameters now work better together

Misc:

Updated error locations. We've spent a lot of time auditing our error locations and trying to move them closer to the actual errors.
esproposal.decorators option to tell Flow to parse and ignore decorators
Bunch of updates to the libraries
Some perf work
Test output is colorized

v0.14.0

Likely to cause new Flow errors:

Assignment now triggers a refinement. If you have a variable that is a ?string and you assign 'hello' to it, Flow refines its type to 'string'.

Likely to fix old Flow errors:

We now treat missing type parameters as any. For example, previously ReactElement was treated as ReactElement<*, *, *>. Now it's treated as ReactElement<any, any, any>.

Misc:

Basic unsafe support for getters & setters, gated behind the config option unsafe.enable_getters_and_setters
Support for block comments inside of Flow's comment syntax
Disable by default munging of class property names that start with an underscore, with an option to enable it
Lots of small internal fixes and merged PRs
Basic semver support for the .flowconfig version
Support for declare type in lib files
Type annotations are now opaque - other types will not flow through them
You can configure the tmp dir that Flow uses

v0.13.1

Likely to cause new Flow errors:

Restricted + and += to only allow strings and numbers, and no longer implicitly cast objects, booleans, null or undefined. Use String(x) to explicitly cast these values.
Fixed a few bugs where types shared between modules may have lost precision or weren't enforced.

Misc:

Added import typeof feature that allows you to import the type of a value export from another module. It is sugar for: import MyThing_tmp from "MyModule"; type MyThing = typeof MyThing_tmp; (except it removes the need for the intermediate MyThing_tmp variable)
Added flow ast command to print a serialized JSON ESTree AST. (Note that this AST does not include types, just syntactic structure for now)
Added support for class expressions
Added support for following symlinks
Added support for number-literal and boolean-literal annotations. (useful for things like enum types and refinements based on tests of equivalence between two variables)
Added support for ES6 binary and octal integer literals
Added support for export type within a CommonJS module that also uses the module.exports = ... pattern
Added support for refining some union types down to their disjoint members
Added support for covariant Promise type parameters
Added improved support for understanding ES5-style imperative class definitions (i.e. via functions + prototypes)
Fixed passing undefined to optional parameters
Fixed return-type tracking for tagged template usage
Fixed an issue where library parse errors would cause the flow server to continuously restart upon initialization without giving an error

v0.12.0

Likely to cause new Flow errors:

Fixed a bug where declarations from libraries which are exported from one module are not checked properly in the module into which that module is imported (e.g. if A.foo() returns a Promise, and module B requires A and calls A.foo(), the return type of A.foo() was not being checked properly)
Fixed enforcement of Object and Function type annotation arity, so that Object<K, V> errors
Restricted valid computed properties, such that only strings and numbers are allowed (notably, disallows null and undefined)

New features:

Added support for for-of and support for Iterable interface(s)
Added support for async/await
Added structural subtyping for interfaces -- anything can be an instance of an interface as long as it looks right
Added support for type annotations of the form typeof x, where x is the name of an in-scope variable
Added a new config option suppress_comment, a regexp which matches against comments and causes any Flow error on the next line to be suppressed. For example, suppress_comment=.*\$FlowFixMe.* will cause /* $FlowFixMe */\nvar x : number = "oops"; to not raise an error.
Added a new config option module.name_mapper, a regexp -> replacement template tuple to be applied to any matching module names before the Flow system resolves the name and looks it up
Added a --color=always|never|auto CLI option, useful when piping to less -R
Added a --one-line CLI option which replaces \n with \\n in multiline error messages, useful when piping to grep

Misc:

Many improvements to library files, especially node and ES6 APIs
Improved warnings on unsupported class members [PR #461]
Added support for export default class
Fixed if (x instanceof Array)
Fixed the type of x && y when x is an array, object or function
Fixed the flow get-def command, especially around imported types
Fixed a bug with == and improved comparison-related error messages
Fixed file watching for individual files included via .flowconfig [includes]
Fixed the build ID, so that the server restarts when accessed from a mismatched client version
Added a new config option log.file which overrides the default log file path

v0.11.0

We are now syncing Flow's commit history to GitHub. No more huge updating diffs. We'll also filter the changelog to the most important things.
Big React refactoring to support ES6 React classes
Do you use any to workaround things that you want to fix later? Well, now you can use $FixMe instead of any and easily grep for these workarounds later.
We now report parsing errors in non-@flow files that you are require()'ing/import'ing
Better error messages and better error message positions
Better error traces, with flow check --traces N, where N is the trace depth
Basic support for Object.freeze()
Assorted fixes and updates to the flow libs
We're trying to be better about commenting the code

v0.10.0

Bump version to 0.10.0
Support import/export type
[PR #412] More fs implementation
Support for static overloads
Fix order of args matched with overloads
Allow methods to have properties
[PR #405] Make interface-defined modules work with CommonJS/ES6Module interop
Refine mixed
Fix typeof null === "object" refinement
[PR #397] Facilitate debug builds
[PR #401] Improve error message clarity in a few places
Remove 'everything is a bool' implicit cast
Fix issues with optional properties during InstanceT ~> ObjT
Fix abnormals in catch blocks

v0.9.2

Fix lowercasing issue where "flow Path/To/Root" became "flow path/to/root"
Fix "My Path/To Flow/flow path/to/root" not autostarting server due to spaces

v0.9.1

Unbreak the command line for "flow path/to/root" (thanks samwgoldman for the report!)

v0.9.0

Bump version to 0.9.0
Add Video-, Audio and TextTrackList (+ dependencies)
Refine switch cases
Better Not_found error
[PR #333] Partial fs module implementation
Update parser to match esprima-fb
[PR #370] Dom canvas context
[PR #393] Symbol should be called statically. Fix #391
Fix havoc_env to copy the for_type value
add bash completion
rewrite all the commands to use CommandSpec
json_of_t
[PR #384] Env printer
[PR #383] Add some build/test artifacts to .gitignore
[PR #377] Add missing HTMLImageElement properties
[PR #367] Re-add merged out oneOfType proptype implementation
Allow users to pin their .flowconfig to a specific version
colorize test failure diffs
Object spread fix
[PR #322] Add type signature for node "url", "querystring" and "path" modules
command parsing library
look for package.json in include paths
clear module errors properly
add verbose mode

v0.8.0

Bump version to 0.8.0
[PR #356] Add React.version to lib/react.js
[PR #331] node http.get() implementation
Fixes for indexers in object types and objects used as dictionaries
Flow server speedup
Support both commas and semicolons in object type patterns
Fix object assign for unresolved objects
Add prerr_endlinef
Fix support for the mixin pattern of specifying super
Incremental typechecking fix
Fix up promises interface definition + more tests
Perf win by loading master_cx once per job
Support for-in of null and undefined
Support typeof x === 'boolean'
Refine true and false literals
Fix refinement unit test
Type declarations for Geolocation interface
[PR #321] Support for React.cloneElement in 0.13
Refine simple assignments (things like while (x = x.parent) { x.doStuff(); })

v0.7.0

Bump version to 0.7.0
Initial support for ES6 import/export (with CommonJS interop)
Updates to CSS/CSSOM interface definitions
propTypes in React components now dictate the concrete type signature for this.props
Show errors in type-at-pos output
Flow now watches include paths specified in .flowconfig for file changes
Interpret x != undefined and x != null as the same refinement
Use Object.prototype.hasOwnProperty() calls as a refinement
Updates to Element and HTMLElement interface definitions

v0.6.0

Bump version to 0.6.0
Also watch for changes in include paths
Fix the Function type to be any function
Add Symbol global name to libs
Support trailing commas in parser
Make methods immutable
Remove tuple array length limit
[PR #315] Implement more PropTypes
Update componentWillReceive spec
Unsuppress library errors

v0.5.0

Bump version to 0.5.0
Add HTMLAnchorElement
[PR #295] Add one of proptype
Update the parser to work with the new esprima-fb & ast-types
[PR #299] Declare prompt function (fixes #204)
[PR #303] Add String.prototype.endsWith()
quick fix for react/es6 notation
extend scope of type params to following bounds
reasonless shortcut
Add React.findDOMNode to lib/react.js
move command documentation into each command
fix path in --help usage example
fix predicate filtering of null and undefined
[PR #292] Types for React synthetic events
basic support for bounded polymorphism
infer falsiness
node haste module support
add Abnormal.string

v0.4.0

** Flow comments should be ready for use **

Bump version to 0.4.0
[PR #259] Add declaration for Document.execCommand()
[PR #265] flow status --json should exit 2 on errors
Fix issue with switch statement breaks
abnormal exit effects of fall-through cases in switch
Let AnyObjT flow to KeyT
invariant(false, ...) always throws
Improve the "Object" annotations
sealed object literals
fix return types which could be undefined
Support type annotations in comments
[PR #263] disable colors when TERM=dumb
Add lint checks for TRUE / FALSE / NULL
class subtyping
array literal inference
Add unsafe_xhp typechecker option
work around false positive conflict markers
Better dumping types for debugging

v0.3.0

** Type casts and import type should be ready for use **

Bump version to 0.3.0
Move haste module name preprocessing into module_js
[PR #260] Add SyntheticEvent to lib/react.js
Add import type to Flow
[Parser] Add esprima tests for typecasts
Add support for a module name preprocessor that can re-write require()d module names
[Parser] Support import types
Allow absolute paths for [libs] and [include]
prepare flow-parser to be published to npm
[PR #247] Resubmit #113 this fixes #155
[PR #246] Fixes #195

v0.2.0

Bump version to 0.2.0
Fix refinement of optional nullable types
Typecast expressions (warning - not added to strip out transform yet)
treat throw as similar to return
Switch from React to react (React still included until facebook/react#3031)
Fix types for sessionStorage and Storage
tighter type for undefined values
no lib paths fix
Fix exporting types for builtin modules
Add Number.isNaN and Number.isSafeInteger type annotations
Optional properties in objects & optional type revamp

v0.1.6

Bump version to 0.1.6
declare modules that redefine exports instead of exporting multiple things
Object spread race condition fix
Fix fallback path to flowlib.tar.gz
[Parser] Type grouping bug fix
Add Object.is
uncomment delete method signatures
Add MAX_SAFE_INTEGER and MIN_SAFE_INTEGER to Number
make param annotations strict upper bounds

v0.1.5

Bump version to 0.1.5
[PR #223] from commonlisp/master
[PR #221] from teppeis/Javascript-to-JavaScript
Better types for React.PropTypes
Make React a module
Check PropTypes as a normal object too
Better support for object call properties
Add range/offset information to Ast.Loc.t
ignored react attributes key & ref passed in spread properties
ignored react attributes key & ref
Add some checks when upper bound object type has indexer
fix type arg arity exception
add traces to .flowconfig
fix error message when undeclared prop is set by jsx
[Parser] Actually test ArrayTypeAnnotation
Fix problem with returns in catch
JSX with spread attributes
Incremental mode fix
Add missing constructors to TypedArray declarations
[Parser] Update Flow's test suite to match updated esprima

v0.1.4

Bump Flow version to 0.1.4
[Flow] Unbreak the open source build
[PR #200] from fmahnke/node_http_interface
[PR #203] from rsolomo/node-net-isip
[PR #201] from unknownexception/patch-1
[Flow] Fill out Map definition
try-catch
missing annotation errors for type aliases
Type refinement fixes for early return

v0.1.3

Handle strict undefined checks.
Add theoretical support to print tuple types properly.
Relax the callback type in Array.prototype.forEach
Add basic autocomplete support for React classes.
[Hack] Kill shell.ml entirely.
Handle spread attributes in JSX syntax.
[Parser] Allow most keywords as types
Add Array.from to lib file
[#161]: String literal keys for object type aliases
Generalize mock module handling under Haste a bit.
Improve autocomplete type printing
misc cleanup
[Parser] Improve type keywords
[Parser] Stop vendoring in ast-types
Add an ocaml wrapper for find path1 ... pathN -name pattern
Restart if a known lib file changes
Add ProgressEvent definition and update two onprogress types
[Hack] Add client time to server start time.
Improve control flow type refinements
[Hack] Reduce dependencies on ParserHeap.
Exit server on config change for now rather than restart to avoid some bugs
[Parser] ArrowFunction object & array patterns must be wrapped with parens
Properly functorize logging code too
[Hack] Make GC parameters configurable.
Refinements on method calls
[Hack] Log whether we are loading from a saved state in INIT_END.
[Hack] Improve error handling for saved states.
[PR #137] from sverrejoh/master
[PR #150] from unknownexception/fix/dom.js
[PR #146] from k-bx/master
[PR #151] from unknownexception/feature/no_flowlib
[PR #144] from rsolomo/node-assert
[PR #191] from Shraymonks/HTMLBaseElement
[PR #145] from rsolomo/node-util
[PR #192] from Shraymonks/HTMLScriptElement
Add ES6 String.prototype.contains
Add Promise.prototype.done() to Flow lib

v0.1.2

[#78] Fallback to index.js if no main attribute in package.json
[#82] Added ES6 functions to the Math object
[#94] Start a node stdlib interface file
[#98] Declare optional parameters according to HTML standard
[#108] Add buffer class to node.js interface
[#115] Some more node std libs
The module system can now be specified in the .flowconfig file
Libraries can now be specified in the .flowconfig file
Multiple library paths can be specified
Added a command to typecheck a file passed on standard input (for IDEs)
Added HTMLInputElement to the standard library
Improvements to type printing in flow suggest and other commands
Fixes to various issues where parser errors were being suppressed

v0.1.1

[Issue #4] Typecheck .jsx files
[Issue #22] Return a nonzero exit code from 'flow check' if there are errors
[IRC report] Autostart the Flow server on all commands
Improve the printing of types in 'flow suggest'
Add a --timeout option to all commands
[PR #27] Clearer error message when 'flow init' hasn't been run
[PR #39] Fix to Emacs bindings
[PR #53] Support node modules that end in .js
[PR #59] Fix example
[PR #65] Fix dependencies in flux-chat example
[PR #66] Add type definitions for HTMLCanvasElement

v0.1.0

Initial release

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