Web/JavaScript/Reference/Global objects/Symbol/asyncIterator

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.

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Description

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

Examples

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) {
        console.log(x);
        // 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.

Specifications

Specification
ECMAScript (ECMA-262)The definition of 'Symbol.asyncIterator' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`asyncIterator`	Chrome Full support 63	Edge Full support 79	Firefox Full support 57	IE No support No	Opera Full support 50	Safari Full support 11.1	WebView Android Full support 63	Chrome Android Full support 63	Firefox Android No support No	Opera Android Full support 46	Safari iOS No support No	Samsung Internet Android Full support 8.0	nodejs Full support 10.0.0

Legend

Full support: Full support
No support: No support