Symbol.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.

The Symbol.asyncIterator static data property represents the well-known symbol Symbol.asyncIterator. The async iterable protocol looks up this symbol for the method that returns the async iterator for an object. In order for an object to be async iterable, it must have an [Symbol.asyncIterator] key.

Try it

const delayedResponses = {
  delays: [500, 1300, 3500],

  wait(delay) {
    return new Promise((resolve) => {
      setTimeout(resolve, delay);
    });
  },

  async *[Symbol.asyncIterator]() {
    for (const delay of this.delays) {
      await this.wait(delay);
      yield `Delayed response for ${delay} milliseconds`;
    }
  },
};

(async () => {
  for await (const response of delayedResponses) {
    console.log(response);
  }
})();

// Expected output: "Delayed response for 500 milliseconds"
// Expected output: "Delayed response for 1300 milliseconds"
// Expected output: "Delayed response for 3500 milliseconds"

Value

The well-known symbol Symbol.asyncIterator.

Property attributes of Symbol.asyncIterator
Writableno
Enumerableno
Configurableno

Examples

User-defined async iterables

You can define your own async iterable by setting the [Symbol.asyncIterator]() property on an object.

js
const myAsyncIterable = {
  async *[Symbol.asyncIterator]() {
    yield "hello";
    yield "async";
    yield "iteration!";
  },
};

(async () => {
  for await (const x of myAsyncIterable) {
    console.log(x);
  }
})();
// Logs:
// "hello"
// "async"
// "iteration!"

When creating an API, remember that async iterables are designed to represent something iterable — like a stream of data or a list —, not to completely replace callbacks and events in most situations.

Built-in async iterables

There is no object in the core JavaScript language that is async iterable. Some web APIs, such as ReadableStream, have the Symbol.asyncIterator method set by default.

Specifications

Specification
ECMAScript® 2025 Language Specification
# sec-symbol.asynciterator

Browser compatibility

Report problems with this compatibility data on GitHub
desktopmobileserver
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
Deno
Node.js
asyncIterator

Legend

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Full support
Full support

See also