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Iterator Helpers

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

Status

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

Champions: Michael Ficarra, Yulia Startsev

This proposal is at Stage 2 of The TC39 Process.

Motivation

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.

Proposal

The proposal introduces a collection of new methods on the Iterator and AsyncIterator prototypes, 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

Added Methods

For Iterators and AsyncIterators we add the following methods:

.map(mapperFn)

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.

Example

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(filtererFn)

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.

Example

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(limit)

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.

Example

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(limit)

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.

Example

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};

.asIndexedPairs()

.asIndexedPairs takes no arguments. It returns an iterator where each value produced by the underlying iterator is paired with a counter, starting at 0 and increasing by 1 with every element produced.

Returns an iterator of pairs.

Example

const abc = ["a", "b", "c"].values();

const result = abc
  .asIndexedPairs();
result.next(); //  {value: [0, "a"], done: false};
result.next(); //  {value: [1, "b"], done: false};
result.next(); //  {value: [2, "c"], done: false};

.flatMap(mapperFn)

.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.

Example

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(reducer [, initialValue ])

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.

Example

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

.toArray()

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.

Example

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

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

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

.forEach(fn)

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

Returns undefined.

Example

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

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

.some(fn)

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.

Example

const iter = [1, 2, 3].values();

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

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

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

.every(fn)

.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.

const iter = [1, 2, 3].values();

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

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

naturals().take(4).every(v => v > 1); // false, first value is 1
naturals().take(4).every(v => v >= 1); // true, acting on a new iterator

.find(fn)

.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(object)

.from 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 }

More Example Usage

Lazy Iteration over sets

Iterating over a set of URLs, asynchronously fetching each, and returning an array of their JSON Output.

const responses = await AsyncIterator.from(urls)
  .map(async (url) => {
    const response = await fetch(url);
    return response.json();
  })
  .toArray();

Example of iterating over a potentially infinite iterator and transforming it to an array in groups of 5.

class ObligatoryCryptocurrencyReference extends Component {
  componentWillMount() {
    const items = ticker() // returns async iterator
      .map((c) => createElement('h2', null, `${c.name}: ${c.price}`))
      .take(5) // only consume 5 items of a potentially infinite iterator
      .toArray() // greedily transform async iterator into array
      .then((data) => this.setState({ data }));
  }

  render() {
    return createElement('div', null, this.state.data);
  }
}

Extending Iterator Prototype

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);

Implementations

Implementation tracking of Iterator Helpers

  • Browsers:
    • [] V8
    • [] SpiderMonkey (feature-flagged on Nightly only)
    • [] JavaScriptCore

Q & A

Why not use Array.from + Array.prototype methods?

All of these methods (except for reduce and toArray) 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.

Prior Art

Method Rust Python npm Itertools C#
all
any
chain
collect
count
cycle
enumerate
filter
filterMap
find
findMap
flatMap
flatten
forEach
last
map
max
min
nth
partition
peekable
position
product
reverse
scan
skip
skipWhile
stepBy
sum
take
takeWhile
unzip
zip
compress
permutations
repeat
slice
starmap
tee
compact
contains
range
reduce
sorted
unique
average
empty
except
intersect
prepend
append

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