The Array.fromAsync()
static method creates a new, shallow-copied Array
instance from an async iterable, iterable, or array-like object.
Syntax
Array.fromAsync(arrayLike)
Array.fromAsync(arrayLike, mapFn)
Array.fromAsync(arrayLike, mapFn, thisArg)
Parameters
arrayLike
- : An async iterable, iterable, or array-like object to convert to an array.
mapFn
- : A function to call on every element of the array. If provided, every value to be added to the array is first passed through this function, and
mapFn
's return value is added to the array instead (after being awaited). The function is called with the following arguments:
- : A function to call on every element of the array. If provided, every value to be added to the array is first passed through this function, and
thisArg
- : Value to use as
this
when executingmapFn
.
- : Value to use as
Return value
A new Promise whose fulfillment value is a new Array instance.
Description
Array.fromAsync()
lets you create arrays from:
- async iterable objects (objects such as and AsyncGenerator); or, if the object is not async iterable,
- iterable objects (objects such as Map and Set); or, if the object is not iterable,
- array-like objects (objects with a
length
property and indexed elements).
Array.fromAsync()
iterates the async iterable in a fashion very similar to for await...of. Array.fromAsync()
is almost equivalent to Array.from in terms of behavior, except the following:
Array.fromAsync()
handles async iterable objects.Array.fromAsync()
returns a Promise that fulfills to the array instance.- If
Array.fromAsync()
is called with a non-async iterable object, each element to be added to the array is first awaited. - If a
mapFn
is provided, its input and output are internally awaited.
Array.fromAsync()
and Promise.all can both turn an iterable of promises into a promise of an array. However, there are two key differences:
Array.fromAsync()
awaits each value yielded from the object sequentially.Promise.all()
awaits all values concurrently.Array.fromAsync()
iterates the iterable lazily, and doesn't retrieve the next value until the current one is settled.Promise.all()
retrieves all values in advance and awaits them all.
Examples
Array from an async iterable
const asyncIterable = (async function* () {
for (let i = 0; i < 5; i++) {
await new Promise((resolve) => setTimeout(resolve, 10 * i));
yield i;
}
})();
Array.fromAsync(asyncIterable).then((array) => console.log(array));
// [0, 1, 2, 3, 4]
Array from a sync iterable
Array.fromAsync(
new Map([
[1, 2],
[3, 4],
]),
).then((array) => console.log(array));
// [[1, 2], [3, 4]]
Array from a sync iterable that yields promises
Array.fromAsync(
new Set([Promise.resolve(1), Promise.resolve(2), Promise.resolve(3)]),
).then((array) => console.log(array));
// [1, 2, 3]
Array from an array-like object of promises
Array.fromAsync({
length: 3,
0: Promise.resolve(1),
1: Promise.resolve(2),
2: Promise.resolve(3),
}).then((array) => console.log(array));
// [1, 2, 3]
Using mapFn
Both the input and output of mapFn
are awaited internally by Array.fromAsync()
.
function delayedValue(v) {
return new Promise((resolve) => setTimeout(() => resolve(v), 100));
}
Array.fromAsync(
[delayedValue(1), delayedValue(2), delayedValue(3)],
(element) => delayedValue(element * 2),
).then((array) => console.log(array));
// [2, 4, 6]
Comparison with Promise.all()
Array.fromAsync()
awaits each value yielded from the object sequentially. Promise.all()
awaits all values concurrently.
function* makeAsyncIterable() {
for (let i = 0; i < 5; i++) {
yield new Promise((resolve) => setTimeout(resolve, 100));
}
}
(async () => {
console.time("Array.fromAsync() time");
await Array.fromAsync(makeAsyncIterable());
console.timeEnd("Array.fromAsync() time");
// Array.fromAsync() time: 503.610ms
console.time("Promise.all() time");
await Promise.all(makeAsyncIterable());
console.timeEnd("Promise.all() time");
// Promise.all() time: 101.728ms
})();
No error handling for sync iterables
Similar to for await...of
, if the object being iterated is a sync iterable, and an error is thrown while iterating, the return()
method of the underlying iterator will not be called, so the iterator is not closed.
function* generatorWithRejectedPromises() {
try {
yield 0;
yield Promise.reject(3);
} finally {
console.log("called finally");
}
}
(async () => {
try {
await Array.fromAsync(generatorWithRejectedPromises());
} catch (e) {
console.log("caught", e);
}
})();
// caught 3
// No "called finally" message
If you need to close the iterator, you need to use a for...of loop instead, and await
each value yourself.
(async () => {
const arr = [];
try {
for (const val of generatorWithRejectedPromises()) {
arr.push(await val);
}
} catch (e) {
console.log("caught", e);
}
})();
// called finally
// caught 3