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https://github.com/tc39/proposal-iterator-helpers
Methods for working with iterators in ECMAScript
https://github.com/tc39/proposal-iterator-helpers
ecmascript-proposal iterators javascript tc39
Last synced: 14 days ago
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Methods for working with iterators in ECMAScript
- Host: GitHub
- URL: https://github.com/tc39/proposal-iterator-helpers
- Owner: tc39
- Created: 2019-01-14T20:57:27.000Z (almost 6 years ago)
- Default Branch: main
- Last Pushed: 2024-08-26T16:20:44.000Z (3 months ago)
- Last Synced: 2024-08-27T17:13:48.110Z (3 months ago)
- Topics: ecmascript-proposal, iterators, javascript, tc39
- Language: HTML
- Homepage: https://tc39.es/proposal-iterator-helpers
- Size: 503 KB
- Stars: 1,289
- Watchers: 73
- Forks: 34
- Open Issues: 4
-
Metadata Files:
- Readme: README.md
Awesome Lists containing this project
README
# Iterator Helpers
> [!IMPORTANT]
> This proposal is now a Stage 4 and will be merged into ECMA-262 as part of https://github.com/tc39/ecma262/pull/3395. This repo is no longer active.
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, Kevin Gibbons
Champions: Michael Ficarra, Yulia Startsev
This proposal is at Stage 4 of [The TC39 Process](https://tc39.es/process-document/).
This proposal formerly contained async as well as sync helpers. The async helpers have been split out to [a separate proposal](https://github.com/tc39/proposal-async-iterator-helpers).
## 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](#prior-art--userland-implementations) already provide these interfaces.
## Proposal
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](./DETAILS.md) for details on semantics decisions.
See this proposal rendered [here](https://tc39.es/proposal-iterator-helpers)
## Added Methods
For Iterators 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
```JavaScript
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
```JavaScript
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
```JavaScript
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
```JavaScript
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(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
```JavaScript
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
```JavaScript
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
```JavaScript
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
```JavaScript
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
```JavaScript
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(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.
```JavaScript
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(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`.
```JavaScript
function* naturals() {
let i = 0;
while (true) {
yield i;
i += 1;
}
}
naturals().find(v => v > 1); // 2
```
### `Iterator.from(object)`
`.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.
```JavaScript
class Iter {
next() {
return { done: false, value: 1 };
}
}
const iter = new Iter();
const wrapper = Iterator.from(iter);
wrapper.next() // { value: 1, done: false }
```
## Iterator helpers and the generator protocol
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.
#### 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.
```js
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);
```
## Q & A
### Why not use Array.from + Array.prototype methods?
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.
### How can I access the new intrinsics?
```js
const IteratorHelperPrototype = Object.getPrototypeOf(Iterator.from([]).take(0));
const WrapForValidIteratorPrototype = Object.getPrototypeOf(Iterator.from({ next(){} }));
```
## Prior Art & Userland implementations
- 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](https://nodejs.org/api/stream.html#readable-streams) streams implement this proposal in its entirety.
| 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`.