AsyncIterator

Baseline Widely available *

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨January 2020⁩.

* Some parts of this feature may have varying levels of support.

AnAsyncIterator object is an object that conforms to theasync iterator protocol by providing anext() method that returns a promise fulfilling to an iterator result object. TheAsyncIterator.prototype object is a hidden global object that all built-in async iterators inherit from. It provides a[Symbol.asyncIterator]() method that returns the async iterator object itself, making the async iterator alsoasync iterable.

Note thatAsyncIterator isnot a global object, although it will be in the future with theasync iterator helpers proposal. TheAsyncIterator.prototype object shared by all built-in async iterators can be obtained with the following code:

const AsyncIteratorPrototype = Object.getPrototypeOf(  Object.getPrototypeOf(Object.getPrototypeOf((async function* () {})())),);

Description

Currently, the only built-in JavaScript async iterator is theAsyncGenerator object returned byasync generator functions. There are some other built-in async iterators in web API, such as the one of aReadableStream.

Each of these async iterators have a distinct prototype object, which defines thenext() method used by the particular async iterator. All of these prototype objects inherit fromAsyncIterator.prototype, which provides a[Symbol.asyncIterator]() method that returns the async iterator object itself, making the async iterator alsoasync iterable.

Note:AsyncIterator.prototype does not implement[Symbol.iterator](), so async iterators are notsync iterable by default.

Instance methods

AsyncIterator.prototype[Symbol.asyncDispose](): Calls and awaits thereturn() method ofthis, if it exists. This implements theasync disposable protocol and allows it to be disposed when used withawait using.
AsyncIterator.prototype[Symbol.asyncIterator](): Returns the async iterator object itself. This allows async iterator objects to also be async iterable.

Examples

Using an async iterator as an async iterable

All built-in async iterators are also async iterable, so you can use them in afor await...of loop:

const asyncIterator = (async function* () {  yield 1;  yield 2;  yield 3;})();(async () => {  for await (const value of asyncIterator) {    console.log(value);  }})();// Logs: 1, 2, 3