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AsyncIterator

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 @@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:

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 am @@asyncIterator method that returns the async iterator object itself, making the async iterator also async iterable.

Note: AsyncIterator.prototype does not implement @@iterator, so async iterators are not sync iterable by default.

Instance methods

AsyncIterator.prototype[@@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:

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