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 a [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 | |
|---|---|
| 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);
}
})();
// 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® 2027 Language Specification # sec-symbol.asynciterator |
Browser compatibility
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asyncIterator
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