JavaScript AsyncIterator : AsyncIterable

An AsyncIterator is an Object that returns a sequence of Promises. Since AsyncIterators are AsyncIterables, you can use for await (const value of iterable) {} to easily loop over the values in an AsyncIterator. Calling an async generator function (async function* () {}) returns an AsyncIterable. You may create your own AsyncIterable object by assigning the Symbol.asyncIterator property to an object with a next() method.

Example:

const sleep = t => new Promise(r => setTimeout(r, t));

const stream = async function*() {
  for (let i = 0; i < 3; i++) {
    await sleep(300 * i);
    yield i;
  }
};

const iterator = stream();
console.dir(await iterator.next());
console.dir(await iterator.next());
console.dir(await iterator.next());
console.dir(await iterator.next());

// for await of loops make it easy to loop over async iterables 
for await (const i of stream()) {
  console.log(i);
}

Run

Results:

Spec

Instance Indexers

this[Symbol.asyncIterator] : Function<AsyncIterator>

Returns a method that returns this.

Example:

const sleep = t => new Promise(r => setTimeout(r, t));

const stream = async function*() {
  for (let i = 0; i < 3; i++) {
    await sleep(300 * i);
    yield i;
  }
};

const iterator = stream();

var iteratorIterator = iterator[Symbol.asyncIterator]();

console.log(iterator === iteratorIterator);
console.log();

for await (const x of stream()) {
  console.log(x);
}
console.log();

// Since AsyncIterators are also AsyncIterables, you can use for-await-of loops with them.
for await (const x of iterator) {
  console.log(x);
}

Run

Results:

Instance Methods

next([yieldValue : Object]) : Promise

Promise return type: { value : Object, done : Boolean }

return([yieldValue : Object]) : Promise

throw([rejectReason : Object]) : Promise

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