The Symbol.asyncIterator well-known symbol specifies the default AsyncIterator for an object. If this property is set on an object, it is an async iterable and can be used in a for await...of loop.


The Symbol.asyncIterator symbol is a builtin symbol that is used to access an object's @@asyncIterator method. In order for an object to be async iterable, it must have a Symbol.asyncIterator key.

Property attributes of Symbol.asyncIterator
Writable no
Enumerable no
Configurable no


User-defined Async Iterables

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

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

(async () => {
    for await (const x of myAsyncIterable) {
        // expected output:
        //    "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 are currently no built-in JavaScript objects that have the [Symbol.asyncIterator] key set by default. However, WHATWG Streams are set to be the first built-in object to be async iterable, with [Symbol.asyncIterator] recently landing in the spec.


Browser compatibilityUpdate compatibility data on GitHub

Chrome Edge Firefox Internet Explorer Opera Safari
asyncIterator 63 79 57 No 50 11.1
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
asyncIterator 63 63 No 46 No 8.0
asyncIterator 10.0.0

See also

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