A proposal for several interfaces that will help with general usage and consumption of iterators in ECMAScript.

Authors: Gus Caplan, Michael Ficarra, Adam Vandolder, Jason Orendorff, Kevin Gibbons

Champions: Michael Ficarra, Yulia Startsev

This proposal is at Stage 4 of The TC39 Process.

This proposal formerly contained async as well as sync helpers. The async helpers have been split out to a separate proposal.

Iterators are a useful way to represent large or possibly infinite enumerable data sets. However, they lack helpers which make them as easy to use as Arrays and other finite data structures, which results in certain problems that could be better represented by iterators being expressed in Arrays, or using libraries to introduce the necessary helpers. Many libraries and languages already provide these interfaces.

The proposal introduces a collection of new methods on the Iterator prototype, to allow general usage and consumption of iterators. For specifics on the implemented methods, please refer to the specification.

See DETAILS.md for details on semantics decisions.

See this proposal rendered here

For Iterators we add the following methods:

map takes a function as an argument. It allows users to apply a function to every element returned from an iterator.

Returns an iterator of the values with the map function applied.

function* naturals() {
  let i = 0;
  while (true) {
    yield i;
    i += 1;
  }
}

const result = naturals()
  .map(value => {
    return value * value;
  });
result.next(); //  {value: 0, done: false};
result.next(); //  {value: 1, done: false};
result.next(); //  {value: 4, done: false};

filter takes a function as an argument. It allows users to skip values from an iterator which do not pass a filter function.

Returns an iterator of values from the original iterator that pass the filter.

function* naturals() {
  let i = 0;
  while (true) {
    yield i;
    i += 1;
  }
}

const result = naturals()
  .filter(value => {
    return value % 2 == 0;
  });
result.next(); //  {value: 0, done: false};
result.next(); //  {value: 2, done: false};
result.next(); //  {value: 4, done: false};

take takes an integer as an argument. It returns an iterator that produces, at most, the given number of elements produced by the underlying iterator.

Returns an iterator with items from the original iterator from 0 until the limit.

function* naturals() {
  let i = 0;
  while (true) {
    yield i;
    i += 1;
  }
}

const result = naturals()
  .take(3);
result.next(); //  {value: 0, done: false};
result.next(); //  {value: 1, done: false};
result.next(); //  {value: 2, done: false};
result.next(); //  {value: undefined, done: true};

drop takes an integer as an argument. It skips the given number of elements produced by the underlying iterator before itself producing any remaining elements.

Returns an iterator of items after the limit.

function* naturals() {
  let i = 0;
  while (true) {
    yield i;
    i += 1;
  }
}

const result = naturals()
  .drop(3);
result.next(); //  {value: 3, done: false};
result.next(); //  {value: 4, done: false};
result.next(); //  {value: 5, done: false};

.flatMap takes a mapping function as an argument. It returns an iterator that produces all elements of the iterators produced by applying the mapping function to the elements produced by the underlying iterator.

Returns an iterator of flat values.

const sunny = ["It's Sunny in", "", "California"].values();

const result = sunny
  .flatMap(value => value.split(" ").values());
result.next(); //  {value: "It's", done: false};
result.next(); //  {value: "Sunny", done: false};
result.next(); //  {value: "in", done: false};
result.next(); //  {value: "", done: false};
result.next(); //  {value: "California", done: false};
result.next(); //  {value: undefined, done: true};

reduce takes a function and an optional initial value as an argument. It allows users to apply a function to every element returned from an iterator, while keeping track of the most recent result of the reducer (the memo). For the first element, the given initial value is used as the memo.

Returns a value (in the example, a number) of the type returned to the reducer function.

function* naturals() {
  let i = 0;
  while (true) {
    yield i;
    i += 1;
  }
}

const result = naturals()
  .take(5)
  .reduce((sum, value) => {
    return sum + value;
  }, 3);

result // 13

When you have a non-infinite iterator which you wish to transform into an array, you can do so with the builtin toArray method.

Returns an Array containing the values from the iterator.

function* naturals() {
  let i = 0;
  while (true) {
    yield i;
    i += 1;
  }
}

const result = naturals()
  .take(5)
  .toArray();

result // [0, 1, 2, 3, 4]

For using side effects with an iterator, you can use the .forEach builtin method, which takes as an argument a function.

Returns undefined.

const log = [];
const fn = (value) => log.push(value);
const iter = [1, 2, 3].values();

iter.forEach(fn);
console.log(log.join(", ")) // "1, 2, 3"

To check if any value in the iterator matches a given predicate, .some can be used. It takes as an argument a function which returns true or false.

Returns a boolean which is true if any element returned true when fn was called on it. The iterator is consumed when some is called.

function* naturals() {
  let i = 0;
  while (true) {
    yield i;
    i += 1;
  }
}

const iter = naturals().take(4);

iter.some(v => v > 1); // true
iter.some(v => true); // false, iterator is already consumed.

naturals().take(4).some(v => v > 1); // true
naturals().take(4).some(v => v == 1); // true, acting on a new iterator

.every takes a function which returns a boolean as an argument. It is used to check if every value generated by the iterator passes the test function.

Returns a boolean.

function* naturals() {
  let i = 0;
  while (true) {
    yield i;
    i += 1;
  }
}

const iter = naturals().take(10);

iter.every(v => v >= 0); // true
iter.every(v => false); // true, iterator is already consumed.

naturals().take(4).every(v => v > 0); // false, first value is 0
naturals().take(4).every(v => v >= 0); // true, acting on a new iterator

.find takes a function as an argument. It is used to find the first element in an iterator that matches.

Can be used without take on infinite iterators.

Returns the found element, or undefined if no element matches fn.

function* naturals() {
  let i = 0;
  while (true) {
    yield i;
    i += 1;
  }
}

naturals().find(v => v > 1); // 2

.from is a static method (unlike the others listed above) which takes an object as an argument. This method allows wrapping "iterator-like" objects with an iterator.

Returns the object if it is already an iterator, returns a wrapping iterator if the passed object implements a callable @@iterator property.

class Iter {
  next() {
    return { done: false, value: 1 };
  }
}

const iter = new Iter();
const wrapper = Iterator.from(iter);

wrapper.next() // { value: 1, done: false }

The generator protocol facilitates coordination between a producer and a consumer, which is necessarily broken by iteration-based transforms. There is no way to properly preserve or re-establish this coordination. We've taken the philosophy that any iterators produced by the helpers this proposal adds only implement the iterator protocol and make no attempt to support generators which use the remainder of the generator protocol. Specifically, such iterators do not implement .throw and do not forward the parameter of .next or .return to an underlying or "source" iterator.

With this proposal, it will be easier to extend the IteratorPrototype for a custom class. See the below example for the previous implementation compared to the new one.

const MyIteratorPrototype = {
  next() {},
  throw() {},
  return() {},

  // but we don't properly implement %IteratorPrototype%!!!
};

// Previously...
// Object.setPrototypeOf(MyIteratorPrototype,
//   Object.getPrototypeOf(Object.getPrototypeOf([][Symbol.iterator]())));

Object.setPrototypeOf(MyIteratorPrototype, Iterator.prototype);

All of the iterator-producing methods in this proposal are lazy. They will only consume the iterator when they need the next item from it. Especially for iterators that never end, this is key. Without generic support for any form of iterator, different iterators have to be handled differently.

const IteratorHelperPrototype = Object.getPrototypeOf(Iterator.from([]).take(0));
const WrapForValidIteratorPrototype = Object.getPrototypeOf(Iterator.from({ next(){} }));

• https://www.npmjs.com/package/itertools
• https://www.npmjs.com/package/lodash
• https://docs.python.org/3/library/itertools.html
• https://github.com/more-itertools/more-itertools
• https://docs.rs/itertools/
• https://doc.rust-lang.org/std/iter/trait.Iterator.html
• https://www.boost.org/doc/libs/1_66_0/libs/iterator/doc/index.html
• https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable
• https://github.com/ReactiveX/IxJS
• https://www.npmjs.com/package/ballvalve
• https://github.com/zloirock/core-js#iterator-helpers
• Node.js Readable streams implement this proposal in its entirety.

Note: The method names are combined, such as toArray and collect.