Array.fromAsync for JavaScript

ECMAScript Stage-2 Proposal. J. S. Choi, 2021.

• Specification available
• Experimental polyfills (do not use in production code yet):
  • array-from-async
  • core-js

Why an Array.fromAsync method

Since its standardization in JavaScript, Array.from has become one of Array’s most frequently used built-in methods. However, no similar functionality exists for async iterators.

Such functionality would be useful for dumping the entirety of an async iterator into a single data structure, especially in unit tests or in command-line interfaces. (Several real-world examples are included in a following section.)

There is an it-all NPM library that performs only this task and which gets about 50,000 weekly downloads daily. This of course does not include any code that uses ad-hoc for await–of loops with empty arrays:

const arr = [];
for await (const item of asyncItems) {
  arr.push(item);
}

Further demonstrating the demand for such functionality, several Stack Overflow questions have been asked by various developers, asking how to convert async iterators to arrays.

There are several real-world examples listed later in this explainer.

Description

(A formal draft specification is available.)

Array.fromAsync is to for await
as Array.from is to for.

Similarly to Array.from, Array.fromAsync would be a static method of the Array built-in class, with one required argument and two optional arguments: (items, mapfn, thisArg).

Async-iterable inputs

But, instead of converting a sync iterable to an array, Array.fromAsync can convert an async iterable to a promise that (if everything goes well) will resolve to a new array. Before the promise resolves, it will create an async iterator from the input, lazily iterate over it, and add each yielded value to the new array. (The promise is immediately returned after the Array.fromAsync function call, no matter what.)

async function * asyncGen (n) {
  for (let i = 0; i < n; i++)
    yield i * 2;
}

// `arr` will be `[0, 2, 4, 6]`.
const arr = [];
for await (const v of asyncGen(4)) {
  arr.push(v);
}

// This is equivalent.
const arr = await Array.fromAsync(asyncGen(4));

Sync-iterable inputs

If the argument is a sync iterable (and not an async iterable), then the return value is still a promise that will resolve to an array. If the sync iterator yields promises, then each yielded promise is awaited before its value is added to the new array. (Values that are not promises are also awaited for one microtick to prevent Zalgo.) All of this matches the behavior of for await.

function * genPromises (n) {
  for (let i = 0; i < n; i++)
    yield Promise.resolve(i * 2);
}

// `arr` will be `[ 0, 2, 4, 6 ]`.
const arr = [];
for await (const v of genPromises(4)) {
  arr.push(v);
}

// This is equivalent.
const arr = await Array.fromAsync(genPromises(4));

Also like for await, when given a sync-but-not-async iterable input, then Array.fromAsync will not handle any yielded promises that reject. This is because Array.fromAsync lazily iterates over its input, so it cannot attach any handlers to its input’s promise values. For more information, see § Errors.

Non-iterable array-like inputs

Array.fromAsync’s valid inputs are a superset of Array.from’s valid inputs. This includes non-iterable array-likes: objects that have a length property as well as indexed elements (similarly to Array.prototype.values). The return value is still a promise that will resolve to an array. If the array-like object’s elements are promises, then each accessed promise is awaited before its value is added to the new array.

One TC39 representative’s opinion: “[Array-likes are] very much not obsolete, and it’s very nice that things aren’t forced to implement the iterator protocol to be transformable into an Array.”

const arrLike = {
  length: 4,
  0: Promise.resolve(0),
  1: Promise.resolve(2),
  2: Promise.resolve(4),
  3: Promise.resolve(6),
}

// `arr` will be `[ 0, 2, 4, 6 ]`.
const arr = [];
for await (const v of Array.from(arrLike)) {
  arr.push(v);
}

// This is equivalent.
const arr = await Array.fromAsync(arrLike);

As with sync iterables, when given a non-iterable input, then Array.fromAsync will not handle any yielded promises that reject. This is because Array.fromAsync lazily iterates over its input, so it cannot attach any handlers to its input’s promise values. For more information, see § Errors.

Generic factory method

Array.fromAsync is a generic factory method. It does not require that its this receiver be the Array constructor. fromAsync can be transferred to or inherited by any other constructor with a single numeric parameter. In that case, the final result will be the data structure created by that constructor (with 0 as its argument), and with each value yielded by the input being assigned to the data structure’s numeric properties. (Symbol.species is not involved at all.) If the this receiver is not a constructor, then fromAsync creates an array as usual. This matches the behavior of Array.from.

async function * asyncGen (n) {
  for (let i = 0; i < n; i++)
    yield i * 2;
}
function Data (n) {}
Data.from = Array.from;
Data.fromAsync = Array.fromAsync;

// d will be a `new Data(0)`, with its `0` property assigned to `0`, its `1`
// property assigned to `2`, etc.
const d = new Data(0); let i = 0;
for await (const v of asyncGen(4)) {
  d[i] = v;
}

// This is equivalent.
const d = await Data.fromAsync(asyncGen(4));

Optional parameters

Array.fromAsync has two optional parameters: mapfn and thisArg.

mapfn is a mapping callback, which is called on each value yielded from the input (and awaited if it came from a synchronous input), along with its index integer (starting from 0). Each result of the mapping callback is, in turn, awaited then added to the array. By default, the callback is essentially an identity function.

thisArg is a this-binding receiver value for the mapping callback. By default, this is undefined. These optional parameters match the behavior of Array.from. Their exclusion would be surprising to developers who are already used to Array.from.

async function * asyncGen (n) {
  for (let i = 0; i < n; i++)
    yield i * 2;
}

// `arr` will be `[ 0, 4, 16, 36 ]`.
const arr = [];
for await (const v of asyncGen(4)) {
  arr.push(v ** 2);
}

// This is equivalent.
const arr = await Array.fromAsync(asyncGen(4), v =>
  v ** 2);

Errors

Like other promise-based APIs, Array.fromAsync will always immediately return a promise. Array.fromAsync will never synchronously throw an error and summon Zalgo.

If Array.fromAsync’s input throws an error while creating its async or sync iterator, then Array.fromAsync’s returned promise will reject with that error.

const err = new Error;
const badIterable = { [Symbol.iterator] () { throw err; } };

// This creates a promise that will reject with `err`.
Array.fromAsync(badIterable);

If Array.fromAsync’s input is iterable but the input’s iterator throws while iterating, then Array.fromAsync’s returned promise will reject with that error.

const err = new Error;
function * genError () { throw err; }

// This creates a promise that will reject with `err`.
Array.fromAsync(genError());

const err = new Error;
function * genErrorAsync () { throw err; }

// This creates a promise that will reject with `err`.
Array.fromAsync(genErrorAsync());

If Array.fromAsync’s input is sync (i.e., the input is not an async iterable), and if one of the input’s values is a promise that eventually rejects or has rejected, then Array.fromAsync’s returned promise will reject with that error.

const err = new Error;
const rejection = Promise.reject(err);
function * genZeroThenRejection () {
  yield 0;
  yield rejection;
}

// This creates a promise that will reject with `err`. However, `rejection`
// itself will not be handled by Array.fromAsync.
Array.fromAsync(genZeroThenRejection());

const err = new Error;
const arrLikeWithRejection = {
  length: 2,
  0: 0,
  1: Promise.reject(err),
};

// This creates a promise that will reject with `err`.
Array.fromAsync(arrLikeWithRejection);

However, like for await, in this case Array.fromAsync will not handle any yielded rejecting promises. This is because Array.fromAsync lazily iterates over its input, so it cannot attach any handlers to its input’s promise values.

The creator of the rejecting promise is expected to synchronously attach a rejection handler when the promise is created, as usual:

const err = new Error;
// The creator of the rejecting promise attaches a rejection handler.
const rejection = Promise.reject(err).catch(console.error);
function * genZeroThenRejection () {
  yield 0;
  yield rejection;
}

// This still creates a promise that will reject with `err`. `err` will also
// separately be printed to the console due to the rejection handler.
Array.fromAsync(genZeroThenRejection());

const err = new Error;
const arrLikeWithRejection = {
  length: 2,
  0: 0,
  1: Promise.reject(err),
};

// This still creates a promise that will reject with `err`. `err` will also
// separately be printed to the console due to the rejection handler.
Array.fromAsync(arrLikeWithRejection);

Alternatively, the user of the promises can switch from Array.fromAsync to Promise.all. Promise.all would change the control flow from lazy sync iteration (with sequential awaiting) to eager sync iteration (with parallel awaiting), allowing the handling of any rejection in the input.

const err = new Error;
const rejection = Promise.reject(err);
function * genZeroThenRejection () {
  yield 0;
  yield rejection;
}

// Creates a promise that will reject with `err`. Unlike Array.fromAsync,
// Promise.all will handle the `rejection`.
Promise.all(genZeroThenRejection());

const err = new Error;
const arrLikeWithRejection = {
  length: 2,
  0: 0,
  1: Promise.reject(err),
};

// Creates a promise that will reject with `err`. Unlike Array.fromAsync,
// Promise.all will handle the `rejection`.
Promise.all(Array.from(arrLikeWithRejection));

If Array.fromAsync’s input has at least one value, and Array.fromAsync’s mapping callback throws an error when given any of those values, then Array.fromAsync’s returned promise will reject with the first such error.

const err = new Error;
function badCallback () { throw err; }

// This creates a promise that will reject with `err`.
Array.fromAsync([ 0 ], badCallback);

If Array.fromAsync’s input is null or undefined, or if Array.fromAsync’s mapping callback is neither undefined nor callable, then Array.fromAsync’s returned promise will reject with a TypeError.

// These create promises that will reject with TypeErrors.
Array.fromAsync(null);
Array.fromAsync([], 1);

Other proposals

Relationship with iterator-helpers

The iterator-helpers proposal has toArray, which works with both sync and async iterables.

Array.from(gen())
gen().toArray()
Array.fromAsync(asyncGen())
asyncGen().toArray()

toArray overlaps with both Array.from and Array.fromAsync. This is okay. They can coexist. If we have to choose between having toArray and having fromAsync, then we should choose fromAsync. We already have Array.from. We should match the existing language precedent.

A co-champion of iterable-helpers agrees that we should have both or that we should prefer Array.fromAsync: “I remembered why it’s better for a buildable structure to consume an iterable than for an iterable to consume a buildable protocol. Sometimes building something one element at a time is the same as building it [more than one] element at a time, but sometimes it could be slow to build that way or produce a structure with equivalent semantics but different performance properties.”

TypedArray.fromAsync, Set.fromAsync, Object.fromEntriesAsync, etc.

The following built-ins also resemble Array.from:

TypedArray.from()
new Set
Object.fromEntries()
new Map

We are deferring any async versions of these methods to future proposals. See issue #8 and proposal-setmap-offrom.

Async spread operator

In the future, standardizing an async spread operator (like [ 0, await ...v ]) may be useful. This proposal leaves that idea to a separate proposal.

Records and tuples

The record/tuple proposal puts forward two new data types with APIs that respectively resemble those of Array and Object. The Tuple constructor, too, would probably need an fromAsync method. Whether the Record constructor gets a fromEntriesAsync method will depend on whether Object.fromEntriesAsync will also be added in a separate proposal.

Real-world examples

Only minor formatting changes have been made to the status-quo examples.

Status quo	With Array.fromAsync
const all = require('it-all'); // Add the default assets to the repo. const results = await all( addAll( globSource(initDocsPath, { recursive: true, }), { preload: false }, ), ); const dir = results .filter(file => file.path === 'init-docs') .pop() print('to get started, enter:\n'); print( `\tjsipfs cat` + `/ipfs/${dir.cid}/readme\n`, ); From ipfs-core/src/runtime/init-assets-nodejs.js.	// Add the default assets to the repo. const results = await Array.fromAsync( addAll( globSource(initDocsPath, { recursive: true, }), { preload: false }, ), ); const dir = results .filter(file => file.path === 'init-docs') .pop() print('to get started, enter:\n'); print( `\tjsipfs cat` + `/ipfs/${dir.cid}/readme\n`, );
const all = require('it-all'); const results = await all( node.contentRouting .findProviders('a cid'), ); expect(results) .to.be.an('array') .with.lengthOf(1) .that.deep.equals([result]); From js-libp2p/test/content-routing/content-routing.node.js.	const results = await Array.fromAsync( node.contentRouting .findProviders('a cid'), ); expect(results) .to.be.an('array') .with.lengthOf(1) .that.deep.equals([result]);
async function toArray(items) { const result = []; for await (const item of items) { result.push(item); } return result; } it('empty-pipeline', async () => { const pipeline = new Pipeline(); const result = await toArray( pipeline.execute( [ 1, 2, 3, 4, 5 ])); assert.deepStrictEqual( result, [ 1, 2, 3, 4, 5 ], ); }); From node-httptransfer/test/generator/pipeline.test.js.	it('empty-pipeline', async () => { const pipeline = new Pipeline(); const result = await Array.fromAsync( pipeline.execute( [ 1, 2, 3, 4, 5 ])); assert.deepStrictEqual( result, [ 1, 2, 3, 4, 5 ], ); });