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
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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