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.

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

Note that AsyncIterator is not a global object, although it will be in the future with the async iterator helpers proposal. The AsyncIterator.prototype object shared by all built-in async iterators can be obtained with the following code:

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

Description

Currently, the only built-in JavaScript async iterator is the AsyncGenerator object returned by async generator functions. There are some other built-in async iterators in web API, such as the one of a ReadableStream.

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

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

Instance methods

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 a for await...of loop:

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

Specifications

Specification
ECMAScript Language Specification
# sec-asynciteratorprototype

Browser compatibility

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See also