Babel Plugin Handbook

This document covers how to create Babel plugins.

This handbook is available in other languages, see the README for a complete list.

Introduction

Babel is a generic multi-purpose compiler for JavaScript. More than that it is a collection of modules that can be used for many different forms of static analysis.

Static analysis is the process of analyzing code without executing it. (Analysis of code while executing it is known as dynamic analysis). The purpose of static analysis varies greatly. It can be used for linting, compiling, code highlighting, code transformation, optimization, minification, and much more.

You can use Babel to build many different types of tools that can help you be more productive and write better programs.

For future updates, follow @thejameskyle on Twitter.

Basics

Babel is a JavaScript compiler, specifically a source-to-source compiler, often called a "transpiler". This means that you give Babel some JavaScript code, Babel modifies the code, and generates the new code back out.

ASTs

Each of these steps involve creating or working with an Abstract Syntax Tree or AST.

Babel uses an AST modified from ESTree, with the core spec located here.

function square(n) {
  return n * n;
}

Check out AST Explorer to get a better sense of the AST nodes. Here is a link to it with the example code above pasted in.

This same program can be represented as a tree like this:

- FunctionDeclaration:
  - id:
    - Identifier:
      - name: square
  - params [1]
    - Identifier
      - name: n
  - body:
    - BlockStatement
      - body [1]
        - ReturnStatement
          - argument
            - BinaryExpression
              - operator: *
              - left
                - Identifier
                  - name: n
              - right
                - Identifier
                  - name: n

Or as a JavaScript Object like this:

{
  type: "FunctionDeclaration",
  id: {
    type: "Identifier",
    name: "square"
  },
  params: [{
    type: "Identifier",
    name: "n"
  }],
  body: {
    type: "BlockStatement",
    body: [{
      type: "ReturnStatement",
      argument: {
        type: "BinaryExpression",
        operator: "*",
        left: {
          type: "Identifier",
          name: "n"
        },
        right: {
          type: "Identifier",
          name: "n"
        }
      }
    }]
  }
}

You'll notice that each level of the AST has a similar structure:

{
  type: "FunctionDeclaration",
  id: {...},
  params: [...],
  body: {...}
}

{
  type: "Identifier",
  name: ...
}

{
  type: "BinaryExpression",
  operator: ...,
  left: {...},
  right: {...}
}

Note: Some properties have been removed for simplicity.

Each of these are known as a Node. An AST can be made up of a single Node, or hundreds if not thousands of Nodes. Together they are able to describe the syntax of a program that can be used for static analysis.

Every Node has this interface:

interface Node {
  type: string;
}

The type field is a string representing the type of Node the object is (e.g. "FunctionDeclaration", "Identifier", or "BinaryExpression"). Each type of Node defines an additional set of properties that describe that particular node type.

There are additional properties on every Node that Babel generates which describe the position of the Node in the original source code.

{
  type: ...,
  start: 0,
  end: 38,
  loc: {
    start: {
      line: 1,
      column: 0
    },
    end: {
      line: 3,
      column: 1
    }
  },
  ...
}

These properties start, end, loc, appear in every single Node.

Stages of Babel

The three primary stages of Babel are parse, transform, generate.

Parse

The parse stage, takes code and outputs an AST. There are two phases of parsing in Babel: Lexical Analysis and Syntactic Analysis.

Lexical Analysis

Lexical Analysis will take a string of code and turn it into a stream of tokens.

You can think of tokens as a flat array of language syntax pieces.

n * n;

[
  { type: { ... }, value: "n", start: 0, end: 1, loc: { ... } },
  { type: { ... }, value: "*", start: 2, end: 3, loc: { ... } },
  { type: { ... }, value: "n", start: 4, end: 5, loc: { ... } },
  ...
]

Each of the types here have a set of properties describing the token:

{
  type: {
    label: 'name',
    keyword: undefined,
    beforeExpr: false,
    startsExpr: true,
    rightAssociative: false,
    isLoop: false,
    isAssign: false,
    prefix: false,
    postfix: false,
    binop: null,
    updateContext: null
  },
  ...
}

Like AST nodes they also have a start, end, and loc.

Syntactic Analysis

Syntactic Analysis will take a stream of tokens and turn it into an AST representation. Using the information in the tokens, this phase will reformat them as an AST which represents the structure of the code in a way that makes it easier to work with.

Transform

The transform stage takes an AST and traverses through it, adding, updating, and removing nodes as it goes along. This is by far the most complex part of Babel or any compiler. This is where plugins operate and so it will be the subject of most of this handbook. So we won't dive too deep right now.

Generate

The code generation stage takes the final AST and turns it back into a string of code, also creating source maps.

Code generation is pretty simple: you traverse through the AST depth-first, building a string that represents the transformed code.

Traversal

When you want to transform an AST you have to traverse the tree recursively.

Say we have the type FunctionDeclaration. It has a few properties: id, params, and body. Each of them have nested nodes.

{
  type: "FunctionDeclaration",
  id: {
    type: "Identifier",
    name: "square"
  },
  params: [{
    type: "Identifier",
    name: "n"
  }],
  body: {
    type: "BlockStatement",
    body: [{
      type: "ReturnStatement",
      argument: {
        type: "BinaryExpression",
        operator: "*",
        left: {
          type: "Identifier",
          name: "n"
        },
        right: {
          type: "Identifier",
          name: "n"
        }
      }
    }]
  }
}

So we start at the FunctionDeclaration and we know its internal properties so we visit each of them and their children in order.

Next we go to id which is an Identifier. Identifiers don't have any child node properties so we move on.

After that is params which is an array of nodes so we visit each of them. In this case it's a single node which is also an Identifier so we move on.

Then we hit body which is a BlockStatement with a property body that is an array of Nodes so we go to each of them.

The only item here is a ReturnStatement node which has an argument, we go to the argument and find a BinaryExpression.

The BinaryExpression has an operator, a left, and a right. The operator isn't a node, just a value, so we don't go to it, and instead just visit left and right.

This traversal process happens throughout the Babel transform stage.

Visitors

When we talk about "going" to a node, we actually mean we are visiting them. The reason we use that term is because there is this concept of a visitor.

Visitors are a pattern used in AST traversal across languages. Simply put they are an object with methods defined for accepting particular node types in a tree. That's a bit abstract so let's look at an example.

const MyVisitor = {
  Identifier() {
    console.log("Called!");
  }
};

// You can also create a visitor and add methods on it later
let visitor = {};
visitor.MemberExpression = function() {};
visitor.FunctionDeclaration = function() {}

Note: Identifier() { ... } is shorthand for Identifier: { enter() { ... } }.

This is a basic visitor that when used during a traversal will call the Identifier() method for every Identifier in the tree.

So with this code the Identifier() method will be called four times with each Identifier (including square).

function square(n) {
  return n * n;
}

path.traverse(MyVisitor);
Called!
Called!
Called!
Called!

These calls are all on node enter. However there is also the possibility of calling a visitor method when on exit.

Imagine we have this tree structure:

- FunctionDeclaration
  - Identifier (id)
  - Identifier (params[0])
  - BlockStatement (body)
    - ReturnStatement (body)
      - BinaryExpression (argument)
        - Identifier (left)
        - Identifier (right)

As we traverse down each branch of the tree we eventually hit dead ends where we need to traverse back up the tree to get to the next node. Going down the tree we enter each node, then going back up we exit each node.

Let's walk through what this process looks like for the above tree.

Enter FunctionDeclaration
- Enter Identifier (id)
  - Hit dead end
- Exit Identifier (id)
- Enter Identifier (params[0])
  - Hit dead end
- Exit Identifier (params[0])
- Enter BlockStatement (body)
  - Enter ReturnStatement (body)
    - Enter BinaryExpression (argument)
      - Enter Identifier (left)
        
        Hit dead end
      - Exit Identifier (left)
      - Enter Identifier (right)
        
        Hit dead end
      - Exit Identifier (right)
    - Exit BinaryExpression (argument)
  - Exit ReturnStatement (body)
- Exit BlockStatement (body)
Exit FunctionDeclaration

So when creating a visitor you have two opportunities to visit a node.

const MyVisitor = {
  Identifier: {
    enter() {
      console.log("Entered!");
    },
    exit() {
      console.log("Exited!");
    }
  }
};

If necessary, you can also apply the same function for multiple visitor nodes by separating them with a | in the method name as a string like Identifier|MemberExpression.

Example usage in the flow-comments plugin

const MyVisitor = {
  "ExportNamedDeclaration|Flow"(path) {}
};

You can also use aliases as visitor nodes (as defined in babel-types).

For example,

Function is an alias for FunctionDeclaration, FunctionExpression, ArrowFunctionExpression, ObjectMethod and ClassMethod.

const MyVisitor = {
  Function(path) {}
};

Paths

An AST generally has many Nodes, but how do Nodes relate to one another? We could have one giant mutable object that you manipulate and have full access to, or we can simplify this with Paths.

A Path is an object representation of the link between two nodes.

For example if we take the following node and its child:

{
  type: "FunctionDeclaration",
  id: {
    type: "Identifier",
    name: "square"
  },
  ...
}

And represent the child Identifier as a path, it looks something like this:

{
  "parent": {
    "type": "FunctionDeclaration",
    "id": {...},
    ....
  },
  "node": {
    "type": "Identifier",
    "name": "square"
  }
}

It also has additional metadata about the path:

{
  "parent": {...},
  "node": {...},
  "hub": {...},
  "contexts": [],
  "data": {},
  "shouldSkip": false,
  "shouldStop": false,
  "removed": false,
  "state": null,
  "opts": null,
  "skipKeys": null,
  "parentPath": null,
  "context": null,
  "container": null,
  "listKey": null,
  "inList": false,
  "parentKey": null,
  "key": null,
  "scope": null,
  "type": null,
  "typeAnnotation": null
}

As well as tons and tons of methods related to adding, updating, moving, and removing nodes, but we'll get into those later.

In a sense, paths are a reactive representation of a node's position in the tree and all sorts of information about the node. Whenever you call a method that modifies the tree, this information is updated. Babel manages all of this for you to make working with nodes easy and as stateless as possible.

Paths in Visitors

When you have a visitor that has a Identifier() method, you're actually visiting the path instead of the node. This way you are mostly working with the reactive representation of a node instead of the node itself.

const MyVisitor = {
  Identifier(path) {
    console.log("Visiting: " + path.node.name);
  }
};

a + b + c;

path.traverse(MyVisitor);
Visiting: a
Visiting: b
Visiting: c

State

State is the enemy of AST transformation. State will bite you over and over again and your assumptions about state will almost always be proven wrong by some syntax that you didn't consider.

Take the following code:

function square(n) {
  return n * n;
}

Let's write a quick hacky visitor that will rename n to x.

let paramName;

const MyVisitor = {
  FunctionDeclaration(path) {
    const param = path.node.params[0];
    paramName = param.name;
    param.name = "x";
  },

  Identifier(path) {
    if (path.node.name === paramName) {
      path.node.name = "x";
    }
  }
};

This might work for the above code, but we can easily break that by doing this:

function square(n) {
  return n * n;
}
n;

The better way to deal with this is recursion. So let's make like a Christopher Nolan film and put a visitor inside of a visitor.

const updateParamNameVisitor = {
  Identifier(path) {
    if (path.node.name === this.paramName) {
      path.node.name = "x";
    }
  }
};

const MyVisitor = {
  FunctionDeclaration(path) {
    const param = path.node.params[0];
    const paramName = param.name;
    param.name = "x";

    path.traverse(updateParamNameVisitor, { paramName });
  }
};

path.traverse(MyVisitor);

Of course, this is a contrived example but it demonstrates how to eliminate global state from your visitors.

Scopes

Next let's introduce the concept of a scope. JavaScript has lexical scoping, which is a tree structure where blocks create new scope.

// global scope

function scopeOne() {
  // scope 1

  function scopeTwo() {
    // scope 2
  }
}

Whenever you create a reference in JavaScript, whether that be by a variable, function, class, param, import, label, etc., it belongs to the current scope.

var global = "I am in the global scope";

function scopeOne() {
  var one = "I am in the scope created by `scopeOne()`";

  function scopeTwo() {
    var two = "I am in the scope created by `scopeTwo()`";
  }
}

Code within a deeper scope may use a reference from a higher scope.

function scopeOne() {
  var one = "I am in the scope created by `scopeOne()`";

  function scopeTwo() {
    one = "I am updating the reference in `scopeOne` inside `scopeTwo`";
  }
}

A lower scope might also create a reference of the same name without modifying it.

function scopeOne() {
  var one = "I am in the scope created by `scopeOne()`";

  function scopeTwo() {
    var one = "I am creating a new `one` but leaving reference in `scopeOne()` alone.";
  }
}

When writing a transform, we want to be wary of scope. We need to make sure we don't break existing code while modifying different parts of it.

We may want to add new references and make sure they don't collide with existing ones. Or maybe we just want to find where a variable is referenced. We want to be able to track these references within a given scope.

A scope can be represented as:

{
  path: path,
  block: path.node,
  parentBlock: path.parent,
  parent: parentScope,
  bindings: [...]
}

When you create a new scope you do so by giving it a path and a parent scope. Then during the traversal process it collects all the references ("bindings") within that scope.

Once that's done, there's all sorts of methods you can use on scopes. We'll get into those later though.

Bindings

References all belong to a particular scope; this relationship is known as a binding.

function scopeOnce() {
  var ref = "This is a binding";

  ref; // This is a reference to a binding

  function scopeTwo() {
    ref; // This is a reference to a binding from a lower scope
  }
}

A single binding looks like this:

{
  identifier: node,
  scope: scope,
  path: path,
  kind: 'var',

  referenced: true,
  references: 3,
  referencePaths: [path, path, path],

  constant: false,
  constantViolations: [path]
}

With this information you can find all the references to a binding, see what type of binding it is (parameter, declaration, etc.), lookup what scope it belongs to, or get a copy of its identifier. You can even tell if it's constant and if not, see what paths are causing it to be non-constant.

Being able to tell if a binding is constant is useful for many purposes, the largest of which is minification.

function scopeOne() {
  var ref1 = "This is a constant binding";

  becauseNothingEverChangesTheValueOf(ref1);

  function scopeTwo() {
    var ref2 = "This is *not* a constant binding";
    ref2 = "Because this changes the value";
  }
}

API

Babel is actually a collection of modules. In this section we'll walk through the major ones, explaining what they do and how to use them.

Note: This is not a replacement for detailed API documentation, which is available here.

`babel-parser`

Started as a fork of Acorn, the Babel Parser is fast, simple to use, has plugin-based architecture for non-standard features (as well as future standards).

First, let's install it.

$ npm install --save @babel/parser

Let's start by simply parsing a string of code:

import parser from "@babel/parser";

const code = `function square(n) {
  return n * n;
}`;

parser.parse(code);
// Node {
//   type: "File",
//   start: 0,
//   end: 38,
//   loc: SourceLocation {...},
//   program: Node {...},
//   comments: [],
//   tokens: [...]
// }

We can also pass options to parse() like so:

parser.parse(code, {
  sourceType: "module", // default: "script"
  plugins: ["jsx"] // default: []
});

sourceType can either be "module" or "script" which is the mode that the Babel Parser should parse in. "module" will parse in strict mode and allow module declarations, "script" will not.

Note: sourceType defaults to "script" and will error when it finds import or export. Pass sourceType: "module" to get rid of these errors.

Since the Babel Parser is built with a plugin-based architecture, there is also a plugins option which will enable the internal plugins. Note that the Babel Parser has not yet opened this API to external plugins, although may do so in the future.

To see a full list of plugins, see the Babel parser docs.

`babel-traverse`

The Babel Traverse module maintains the overall tree state, and is responsible for replacing, removing, and adding nodes.

Install it by running:

$ npm install --save @babel/traverse

We can use it alongside to traverse and update nodes:

import parser from "@babel/parser";
import traverse from "@babel/traverse";

const code = `function square(n) {
  return n * n;
}`;

const ast = parser.parse(code);

traverse(ast, {
  enter(path) {
    if (
      path.node.type === "Identifier" &&
      path.node.name === "n"
    ) {
      path.node.name = "x";
    }
  }
});

`babel-types`

Babel Types is a Lodash-esque utility library for AST nodes. It contains methods for building, validating, and converting AST nodes. It's useful for cleaning up AST logic with well thought out utility methods.

You can install it by running:

$ npm install --save @babel/types

Then start using it:

import traverse from "@babel/traverse";
import * as t from "@babel/types";

traverse(ast, {
  enter(path) {
    if (t.isIdentifier(path.node, { name: "n" })) {
      path.node.name = "x";
    }
  }
});

Definitions

Babel Types has definitions for every single type of node, with information on what properties belong where, what values are valid, how to build that node, how the node should be traversed, and aliases of the Node.

A single node type definition looks like this:

defineType("BinaryExpression", {
  builder: ["operator", "left", "right"],
  fields: {
    operator: {
      validate: assertValueType("string")
    },
    left: {
      validate: assertNodeType("Expression")
    },
    right: {
      validate: assertNodeType("Expression")
    }
  },
  visitor: ["left", "right"],
  aliases: ["Binary", "Expression"]
});

Builders

You'll notice the above definition for BinaryExpression has a field for a builder.

builder: ["operator", "left", "right"]

This is because each node type gets a builder method, which when used looks like this:

t.binaryExpression("*", t.identifier("a"), t.identifier("b"));

Which creates an AST like this:

{
  type: "BinaryExpression",
  operator: "*",
  left: {
    type: "Identifier",
    name: "a"
  },
  right: {
    type: "Identifier",
    name: "b"
  }
}

Which when printed looks like this:

a * b

Builders will also validate the nodes they are creating and throw descriptive errors if used improperly. Which leads into the next type of method.

Validators

The definition for BinaryExpression also includes information on the fields of a node and how to validate them.

fields: {
  operator: {
    validate: assertValueType("string")
  },
  left: {
    validate: assertNodeType("Expression")
  },
  right: {
    validate: assertNodeType("Expression")
  }
}

This is used to create two types of validating methods. The first of which is isX.

t.isBinaryExpression(maybeBinaryExpressionNode);

This tests to make sure that the node is a binary expression, but you can also pass a second parameter to ensure that the node contains certain properties and values.

t.isBinaryExpression(maybeBinaryExpressionNode, { operator: "*" });

There is also the more, ehem, assertive version of these methods, which will throw errors instead of returning true or false.

t.assertBinaryExpression(maybeBinaryExpressionNode);
t.assertBinaryExpression(maybeBinaryExpressionNode, { operator: "*" });
// Error: Expected type "BinaryExpression" with option { "operator": "*" }

Converters

[WIP]

`babel-generator`

Babel Generator is the code generator for Babel. It takes an AST and turns it into code with sourcemaps.

Run the following to install it:

$ npm install --save @babel/generator

Then use it

import parser from "@babel/parser";
import generate from "@babel/generator";

const code = `function square(n) {
  return n * n;
}`;

const ast = parser.parse(code);

generate(ast, {}, code);
// {
//   code: "...",
//   map: "..."
// }

You can also pass options to generate().

generate(ast, {
  retainLines: false,
  compact: "auto",
  concise: false,
  quotes: "double",
  // ...
}, code);

`babel-template`

Babel Template is another tiny but incredibly useful module. It allows you to write strings of code with placeholders that you can use instead of manually building up a massive AST. In computer science, this capability is called quasiquotes.

$ npm install --save @babel/template

import template from "@babel/template";
import generate from "@babel/generator";
import * as t from "@babel/types";

const buildRequire = template(`
  var IMPORT_NAME = require(SOURCE);
`);

const ast = buildRequire({
  IMPORT_NAME: t.identifier("myModule"),
  SOURCE: t.stringLiteral("my-module")
});

console.log(generate(ast).code);

var myModule = require("my-module");

Writing your first Babel Plugin

Now that you're familiar with all the basics of Babel, let's tie it together with the plugin API.

Start off with a function that gets passed the current babel object.

export default function(babel) {
  // plugin contents
}

Since you'll be using it so often, you'll likely want to grab just babel.types like so:

export default function({ types: t }) {
  // plugin contents
}

Then you return an object with a property visitor which is the primary visitor for the plugin.

export default function({ types: t }) {
  return {
    visitor: {
      // visitor contents
    }
  };
};

Each function in the visitor receives 2 arguments: path and state

export default function({ types: t }) {
  return {
    visitor: {
      Identifier(path, state) {},
      ASTNodeTypeHere(path, state) {}
    }
  };
};

Let's write a quick plugin to show off how it works. Here's our source code:

foo === bar;

Or in AST form:

{
  type: "BinaryExpression",
  operator: "===",
  left: {
    type: "Identifier",
    name: "foo"
  },
  right: {
    type: "Identifier",
    name: "bar"
  }
}

We'll start off by adding a BinaryExpression visitor method.

export default function({ types: t }) {
  return {
    visitor: {
      BinaryExpression(path) {
        // ...
      }
    }
  };
}

Then let's narrow it down to just BinaryExpressions that are using the === operator.

visitor: {
  BinaryExpression(path) {
    if (path.node.operator !== "===") {
      return;
    }

    // ...
  }
}

Now let's replace the left property with a new identifier:

BinaryExpression(path) {
  if (path.node.operator !== "===") {
    return;
  }

  path.node.left = t.identifier("sebmck");
  // ...
}

Already if we run this plugin we would get:

sebmck === bar;

Now let's just replace the right property.

BinaryExpression(path) {
  if (path.node.operator !== "===") {
    return;
  }

  path.node.left = t.identifier("sebmck");
  path.node.right = t.identifier("dork");
}

And now for our final result:

sebmck === dork;

Awesome! Our very first Babel plugin.

Transformation Operations

Visiting

Get the Path of Sub-Node

To access an AST node's property you normally access the node and then the property. path.node.property

// the BinaryExpression AST node has properties: `left`, `right`, `operator`
BinaryExpression(path) {
  path.node.left;
  path.node.right;
  path.node.operator;
}

If you need to access the path of that property instead, use the get method of a path, passing in the string to the property.

BinaryExpression(path) {
  path.get('left'); 
}
Program(path) {
  path.get('body.0');
}

You can't current use get on a Container (the body array of a BlockStatement), but you chain the dot syntax instead.

export default function f() {
  return bar;
}

For the example above, if you wanted to get the path corresponding to the return, you could chain the various properties, using a number as the index when traversing the array.

ExportDefaultDeclaration(path) {
  path.get("declaration.body.body.0");
}

Check if a node is a certain type

If you want to check what the type of a node is, the preferred way to do so is:

BinaryExpression(path) {
  if (t.isIdentifier(path.node.left)) {
    // ...
  }
}

You can also do a shallow check for properties on that node:

BinaryExpression(path) {
  if (t.isIdentifier(path.node.left, { name: "n" })) {
    // ...
  }
}

This is functionally equivalent to:

BinaryExpression(path) {
  if (
    path.node.left != null &&
    path.node.left.type === "Identifier" &&
    path.node.left.name === "n"
  ) {
    // ...
  }
}

Check if a path is a certain type

A path has the same methods for checking the type of a node:

BinaryExpression(path) {
  if (path.get('left').isIdentifier({ name: "n" })) {
    // ...
  }
}

is equivalent to doing:

BinaryExpression(path) {
  if (t.isIdentifier(path.node.left, { name: "n" })) {
    // ...
  }
}

Check if an identifier is referenced

Identifier(path) {
  if (path.isReferencedIdentifier()) {
    // ...
  }
}

Alternatively:

Identifier(path) {
  if (t.isReferenced(path.node, path.parent)) {
    // ...
  }
}

Find a specific parent path

Sometimes you will need to traverse the tree upwards from a path until a condition is satisfied.

Call the provided callback with the NodePaths of all the parents. When the callback returns a truthy value, we return that NodePath.

path.findParent((path) => path.isObjectExpression());

If the current path should be included as well:

path.find((path) => path.isObjectExpression());

Find the closest parent function or program:

path.getFunctionParent();

Walk up the tree until we hit a parent node path in a list

path.getStatementParent();

Get Sibling Paths

If a path is in a list like in the body of a Function/Program, it will have "siblings".

Check if a path is part of a list with path.inList
You can get the surrounding siblings with path.getSibling(index),
The current path's index in the container with path.key,
The path's container (an array of all sibling nodes) with path.container
Get the name of the key of the list container with path.listKey

These APIs are used in the transform-merge-sibling-variables plugin used in babel-minify.

var a = 1; // pathA, path.key = 0
var b = 2; // pathB, path.key = 1
var c = 3; // pathC, path.key = 2

export default function({ types: t }) {
  return {
    visitor: {
      VariableDeclaration(path) {
        // if the current path is pathA
        path.inList // true
        path.listKey // "body"
        path.key // 0
        path.getSibling(0) // pathA
        path.getSibling(path.key + 1) // pathB
        path.container // [pathA, pathB, pathC]
        path.getPrevSibling() // path(undefined) *
        path.getNextSibling() // pathB
        path.getAllPrevSiblings() // []
        path.getAllNextSiblings() // [pathB, pathC]
      }
    }
  };
}

path(undefined) is a NodePath where the path.node === undefined

Stopping Traversal

If your plugin needs to not run in a certain situation, the simpliest thing to do is to write an early return.

BinaryExpression(path) {
  if (path.node.operator !== '**') return;
}

If you are doing a sub-traversal in a top level path, you can use 2 provided API methods:

path.skip() skips traversing the children of the current path. path.stop() stops traversal entirely.

outerPath.traverse({
  Function(innerPath) {
    innerPath.skip(); // if checking the children is irrelevant
  },
  ReferencedIdentifier(innerPath, state) {
    state.iife = true;
    innerPath.stop(); // if you want to save some state and then stop traversal, or deopt
  }
});

Manipulation

Replacing a node

BinaryExpression(path) {
  path.replaceWith(
    t.binaryExpression("**", path.node.left, t.numberLiteral(2))
  );
}

  function square(n) {
-   return n * n;
+   return n ** 2;
  }

Replacing a node with multiple nodes

ReturnStatement(path) {
  path.replaceWithMultiple([
    t.expressionStatement(t.stringLiteral("Is this the real life?")),
    t.expressionStatement(t.stringLiteral("Is this just fantasy?")),
    t.expressionStatement(t.stringLiteral("(Enjoy singing the rest of the song in your head)")),
  ]);
}

  function square(n) {
-   return n * n;
+   "Is this the real life?";
+   "Is this just fantasy?";
+   "(Enjoy singing the rest of the song in your head)";
  }

Note: When replacing an expression with multiple nodes, they must be statements. This is because Babel uses heuristics extensively when replacing nodes which means that you can do some pretty crazy transformations that would be extremely verbose otherwise.

Replacing a node with a source string

FunctionDeclaration(path) {
  path.replaceWithSourceString(`function add(a, b) {
    return a + b;
  }`);
}

- function square(n) {
-   return n * n;
+ function add(a, b) {
+   return a + b;
  }

Note: It's not recommended to use this API unless you're dealing with dynamic source strings, otherwise it's more efficient to parse the code outside of the visitor.

Inserting a sibling node

FunctionDeclaration(path) {
  path.insertBefore(t.expressionStatement(t.stringLiteral("Because I'm easy come, easy go.")));
  path.insertAfter(t.expressionStatement(t.stringLiteral("A little high, little low.")));
}

+ "Because I'm easy come, easy go.";
  function square(n) {
    return n * n;
  }
+ "A little high, little low.";

Note: This should always be a statement or an array of statements. This uses the same heuristics mentioned in Replacing a node with multiple nodes.

Inserting into a container

If you want to insert into an AST node that is an array like body. Similar to insertBefore/insertAfter, except that you have to specify the listKey, which is usually body.

ClassMethod(path) {
  path.get('body').unshiftContainer('body', t.expressionStatement(t.stringLiteral('before')));
  path.get('body').pushContainer('body', t.expressionStatement(t.stringLiteral('after')));
}

 class A {
  constructor() {
+   "before"
    var a = 'middle';
+   "after"
  }
 }

Removing a node

FunctionDeclaration(path) {
  path.remove();
}

- function square(n) {
-   return n * n;
- }

Replacing a parent

Just call replaceWith with the parentPath: path.parentPath

BinaryExpression(path) {
  path.parentPath.replaceWith(
    t.expressionStatement(t.stringLiteral("Anyway the wind blows, doesn't really matter to me, to me."))
  );
}

  function square(n) {
-   return n * n;
+   "Anyway the wind blows, doesn't really matter to me, to me.";
  }

Removing a parent

BinaryExpression(path) {
  path.parentPath.remove();
}

  function square(n) {
-   return n * n;
  }

Scope

Checking if a local variable is bound

FunctionDeclaration(path) {
  if (path.scope.hasBinding("n")) {
    // ...
  }
}

This will walk up the scope tree and check for that particular binding.

You can also check if a scope has its own binding:

FunctionDeclaration(path) {
  if (path.scope.hasOwnBinding("n")) {
    // ...
  }
}

Generating a UID

This will generate an identifier that doesn't collide with any locally defined variables.

FunctionDeclaration(path) {
  path.scope.generateUidIdentifier("uid");
  // Node { type: "Identifier", name: "_uid" }
  path.scope.generateUidIdentifier("uid");
  // Node { type: "Identifier", name: "_uid2" }
}

Pushing a variable declaration to a parent scope

Sometimes you may want to push a VariableDeclaration so you can assign to it.

FunctionDeclaration(path) {
  const id = path.scope.generateUidIdentifierBasedOnNode(path.node.id);
  path.remove();
  path.scope.parent.push({ id, init: path.node });
}

- function square(n) {
+ var _square = function square(n) {
    return n * n;
- }
+ };

Rename a binding and its references

FunctionDeclaration(path) {
  path.scope.rename("n", "x");
}

- function square(n) {
-   return n * n;
+ function square(x) {
+   return x * x;
  }

Alternatively, you can rename a binding to a generated unique identifier:

FunctionDeclaration(path) {
  path.scope.rename("n");
}

- function square(n) {
-   return n * n;
+ function square(_n) {
+   return _n * _n;
  }

Plugin Options

If you would like to let your users customize the behavior of your Babel plugin you can accept plugin specific options which users can specify like this:

{
  plugins: [
    ["my-plugin", {
      "option1": true,
      "option2": false
    }]
  ]
}

These options then get passed into plugin visitors through the state object:

export default function({ types: t }) {
  return {
    visitor: {
      FunctionDeclaration(path, state) {
        console.log(state.opts);
        // { option1: true, option2: false }
      }
    }
  }
}

These options are plugin-specific and you cannot access options from other plugins.

Pre and Post in Plugins

Plugins can have functions that are run before or after plugins. They can be used for setup or cleanup/analysis purposes.

export default function({ types: t }) {
  return {
    pre(state) {
      this.cache = new Map();
    },
    visitor: {
      StringLiteral(path) {
        this.cache.set(path.node.value, 1);
      }
    },
    post(state) {
      console.log(this.cache);
    }
  };
}

Enabling Syntax in Plugins

Babel plugins themselves can enable parser plugins so that users don't need to install/enable them. This prevents a parsing error without inheriting the syntax plugin.

export default function({ types: t }) {
  return {
    inherits: require("babel-plugin-syntax-jsx")
  };
}

Throwing a Syntax Error

If you want to throw an error with babel-code-frame and a message:

export default function({ types: t }) {
  return {
    visitor: {
      StringLiteral(path) {
        throw path.buildCodeFrameError("Error message here");
      }
    }
  };
}

The error looks like:

file.js: Error message here
   7 |
   8 | let tips = [
>  9 |   "Click on any AST node with a '+' to expand it",
     |   ^
  10 |
  11 |   "Hovering over a node highlights the \
  12 |    corresponding part in the source code",

Building Nodes

When writing transformations you'll often want to build up some nodes to insert into the AST. As mentioned previously, you can do this using the builder methods in the babel-types package.

The method name for a builder is simply the name of the node type you want to build except with the first letter lowercased. For example if you wanted to build a MemberExpression you would use t.memberExpression(...).

The arguments of these builders are decided by the node definition. There's some work that's being done to generate easy-to-read documentation on the definitions, but for now they can all be found here.

A node definition looks like the following:

defineType("MemberExpression", {
  builder: ["object", "property", "computed"],
  visitor: ["object", "property"],
  aliases: ["Expression", "LVal"],
  fields: {
    object: {
      validate: assertNodeType("Expression")
    },
    property: {
      validate(node, key, val) {
        let expectedType = node.computed ? "Expression" : "Identifier";
        assertNodeType(expectedType)(node, key, val);
      }
    },
    computed: {
      default: false
    }
  }
});

Here you can see all the information about this particular node type, including how to build it, traverse it, and validate it.

By looking at the builder property, you can see the 3 arguments that will be needed to call the builder method (t.memberExpression).

builder: ["object", "property", "computed"],

Note that sometimes there are more properties that you can customize on the node than the builder array contains. This is to keep the builder from having too many arguments. In these cases you need to set the properties manually. An example of this is ClassMethod.

// Example
// because the builder doesn't contain `async` as a property
var node = t.classMethod(
  "constructor",
  t.identifier("constructor"),
  params,
  body
)
// set it manually after creation
node.async = true;

You can see the validation for the builder arguments with the fields object.

fields: {
  object: {
    validate: assertNodeType("Expression")
  },
  property: {
    validate(node, key, val) {
      let expectedType = node.computed ? "Expression" : "Identifier";
      assertNodeType(expectedType)(node, key, val);
    }
  },
  computed: {
    default: false
  }
}

You can see that object needs to be an Expression, property either needs to be an Expression or an Identifier depending on if the member expression is computed or not and computed is simply a boolean that defaults to false.

So we can construct a MemberExpression by doing the following:

t.memberExpression(
  t.identifier('object'),
  t.identifier('property')
  // `computed` is optional
);

Which will result in:

object.property

However, we said that object needed to be an Expression so why is Identifier valid?

Well if we look at the definition of Identifier we can see that it has an aliases property which states that it is also an expression.

aliases: ["Expression", "LVal"],

So since MemberExpression is a type of Expression, we could set it as the object of another MemberExpression:

t.memberExpression(
  t.memberExpression(
    t.identifier('member'),
    t.identifier('expression')
  ),
  t.identifier('property')
)

Which will result in:

member.expression.property

It's very unlikely that you will ever memorize the builder method signatures for every node type. So you should take some time and understand how they are generated from the node definitions.

You can find all of the actual definitions here and you can see them documented here

Best Practices

Create Helper Builders and Checkers

It's pretty simple to extract certain checks (if a node is a certain type) into their own helper functions as well as extracting out helpers for specific node types.

function isAssignment(node) {
  return node && node.operator === opts.operator + "=";
}

function buildAssignment(left, right) {
  return t.assignmentExpression("=", left, right);
}

Avoid traversing the AST as much as possible

Traversing the AST is expensive, and it's easy to accidentally traverse the AST more than necessary. This could be thousands if not tens of thousands of extra operations.

Babel optimizes this as much as possible, merging visitors together if it can in order to do everything in a single traversal.

Merge visitors whenever possible

When writing visitors, it may be tempting to call path.traverse in multiple places where they are logically necessary.

path.traverse({
  Identifier(path) {
    // ...
  }
});

path.traverse({
  BinaryExpression(path) {
    // ...
  }
});

However, it is far better to write these as a single visitor that only gets run once. Otherwise you are traversing the same tree multiple times for no reason.

path.traverse({
  Identifier(path) {
    // ...
  },
  BinaryExpression(path) {
    // ...
  }
});

Do not traverse when manual lookup will do

It may also be tempting to call path.traverse when looking for a particular node type.

const nestedVisitor = {
  Identifier(path) {
    // ...
  }
};

const MyVisitor = {
  FunctionDeclaration(path) {
    path.get('params').traverse(nestedVisitor);
  }
};

However, if you are looking for something specific and shallow, there is a good chance you can manually lookup the nodes you need without performing a costly traversal.

const MyVisitor = {
  FunctionDeclaration(path) {
    path.node.params.forEach(function() {
      // ...
    });
  }
};

Optimizing nested visitors

When you are nesting visitors, it might make sense to write them nested in your code.

const MyVisitor = {
  FunctionDeclaration(path) {
    path.traverse({
      Identifier(path) {
        // ...
      }
    });
  }
};

However, this creates a new visitor object every time FunctionDeclaration() is called. That can be costly, because Babel does some processing each time a new visitor object is passed in (such as exploding keys containing multiple types, performing validation, and adjusting the object structure). Because Babel stores flags on visitor objects indicating that it's already performed that processing, it's better to store the visitor in a variable and pass the same object each time.

const nestedVisitor = {
  Identifier(path) {
    // ...
  }
};

const MyVisitor = {
  FunctionDeclaration(path) {
    path.traverse(nestedVisitor);
  }
};

If you need some state within the nested visitor, like so:

const MyVisitor = {
  FunctionDeclaration(path) {
    var exampleState = path.node.params[0].name;

    path.traverse({
      Identifier(path) {
        if (path.node.name === exampleState) {
          // ...
        }
      }
    });
  }
};

You can pass it in as state to the traverse() method and have access to it on this in the visitor.

const nestedVisitor = {
  Identifier(path) {
    if (path.node.name === this.exampleState) {
      // ...
    }
  }
};

const MyVisitor = {
  FunctionDeclaration(path) {
    var exampleState = path.node.params[0].name;
    path.traverse(nestedVisitor, { exampleState });
  }
};

Being aware of nested structures

Sometimes when thinking about a given transform, you might forget that the given structure can be nested.

For example, imagine we want to lookup the constructor ClassMethod from the Foo ClassDeclaration.

class Foo {
  constructor() {
    // ...
  }
}

const constructorVisitor = {
  ClassMethod(path) {
    if (path.node.name === 'constructor') {
      // ...
    }
  }
}

const MyVisitor = {
  ClassDeclaration(path) {
    if (path.node.id.name === 'Foo') {
      path.traverse(constructorVisitor);
    }
  }
}

We are ignoring the fact that classes can be nested and using the traversal above we will hit a nested constructor as well:

class Foo {
  constructor() {
    class Bar {
      constructor() {
        // ...
      }
    }
  }
}

Unit Testing

There are a few primary ways to test babel plugins: snapshot tests, AST tests, and exec tests. We'll use jest for this example because it supports snapshot testing out of the box. The example we're creating here is hosted in this repo.

First we need a babel plugin, we'll put this in src/index.js.

module.exports = function testPlugin(babel) {
  return {
    visitor: {
      Identifier(path) {
        if (path.node.name === 'foo') {
          path.node.name = 'bar';
        }
      }
    }
  };
};

Snapshot Tests

Next, install our dependencies with npm install --save-dev babel-core jest, and then we can begin writing our first test: the snapshot. Snapshot tests allow us to visually inspect the output of our babel plugin. We give it an input, tell it to make a snapshot, and it saves it to a file. We check in the snapshots into git. This allows us to see when we've affected the output of any of our test cases. It also gives use a diff in pull requests. Of course you could do this with any test framework, but with jest updating the snapshots is as easy as jest -u.

// src/__tests__/index-test.js
const babel = require('babel-core');
const plugin = require('../');

var example = `
var foo = 1;
if (foo) console.log(foo);
`;

it('works', () => {
  const {code} = babel.transform(example, {plugins: [plugin]});
  expect(code).toMatchSnapshot();
});

This gives us a snapshot file in src/__tests__/__snapshots__/index-test.js.snap.

exports[`test works 1`] = `
"
var bar = 1;
if (bar) console.log(bar);"
`;

If we change 'bar' to 'baz' in our plugin and run jest again, we get this:

Received value does not match stored snapshot 1.

    - Snapshot
    + Received

    @@ -1,3 +1,3 @@
     "
    -var bar = 1;
    -if (bar) console.log(bar);"
    +var baz = 1;
    +if (baz) console.log(baz);"

We see how our change to the plugin code affected the output of our plugin, and if the output looks good to us, we can run jest -u to update the snapshot.

AST Tests

In addition to snapshot testing, we can manually inspect the AST. This is a simple but brittle example. For more involved situations you may wish to leverage babel-traverse. It allows you to specify an object with a visitor key, exactly like you use for the plugin itself.

it('contains baz', () => {
  const {ast} = babel.transform(example, {plugins: [plugin]});
  const program = ast.program;
  const declaration = program.body[0].declarations[0];
  assert.equal(declaration.id.name, 'baz');
  // or babelTraverse(program, {visitor: ...})
});

Exec Tests

Here we'll be transforming the code, and then evaluating that it behaves correctly. Note that we're not using assert in the test. This ensures that if our plugin does weird stuff like removing the assert line by accident, the test will still fail.

it('foo is an alias to baz', () => {
  var input = `
    var foo = 1;
    // test that foo was renamed to baz
    var res = baz;
  `;
  var {code} = babel.transform(input, {plugins: [plugin]});
  var f = new Function(`
    ${code};
    return res;
  `);
  var res = f();
  assert(res === 1, 'res is 1');
});

Babel core uses a similar approach to snapshot and exec tests.

`babel-plugin-tester`

This package makes testing plugins easier. If you're familiar with ESLint's RuleTester this should be familiar. You can look at the docs to get a full sense of what's possible, but here's a simple example:

import pluginTester from 'babel-plugin-tester';
import identifierReversePlugin from '../identifier-reverse-plugin';

pluginTester({
  plugin: identifierReversePlugin,
  fixtures: path.join(__dirname, '__fixtures__'),
  tests: {
    'does not change code with no identifiers': '"hello";',
    'changes this code': {
      code: 'var hello = "hi";',
      output: 'var olleh = "hi";',
    },
    'using fixtures files': {
      fixture: 'changed.js',
      outputFixture: 'changed-output.js',
    },
    'using jest snapshots': {
      code: `
        function sayHi(person) {
          return 'Hello ' + person + '!'
        }
      `,
      snapshot: true,
    },
  },
});

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