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AsyncGenerator

The AsyncGenerator object is returned by an async generator function and it conforms to both the async iterable protocol and the async iterator protocol.

Async generator methods always yield Promise objects.

Try it

Constructor

The AsyncGenerator constructor is not available globally. Instances of AsyncGenerator must be returned from async generator functions

async function* createAsyncGenerator() {
  yield await Promise.resolve(1);
  yield await Promise.resolve(2);
  yield await Promise.resolve(3);
}
const asyncGen = createAsyncGenerator();
asyncGen.next()
  .then((res) => console.log(res.value)); // 1
asyncGen.next()
  .then((res) => console.log(res.value)); // 2
asyncGen.next()
  .then((res) => console.log(res.value)); // 3

Instance methods

AsyncGenerator.prototype.next(): Returns a Promise which will be resolved with the given value yielded by the yield expression.
AsyncGenerator.prototype.return(): Acts as if a return statement is inserted in the generator's body at the current suspended position, which finishes the generator and allows the generator to perform any cleanup tasks when combined with a try...finally block.
AsyncGenerator.prototype.throw(): Acts as if a throw statement is inserted in the generator's body at the current suspended position, which informs the generator of an error condition and allows it to handle the error, or perform cleanup and close itself.

Examples

Async generator iteration

The following example iterates over an async generator, logging values 1–6 to the console at decreasing time intervals. Notice how each time a Promise is yielded, but it's automatically resolved within the for await...of loop.

// An async task. Pretend it's doing something more useful
// in practice.
function delayedValue(time, value) {
  return new Promise((resolve /*, reject*/) => {
    setTimeout(() => resolve(value), time);
  });
}

async function* generate() {
  yield delayedValue(2000, 1);
  yield delayedValue(100, 2);
  yield delayedValue(500, 3);
  yield delayedValue(250, 4);
  yield delayedValue(125, 5);
  yield delayedValue(50, 6);
  console.log('All done!');
}

async function main() {
  for await (const value of generate()) {
    console.log('value', value);
  }
}

main()
  .catch((e) => console.error(e));

Specifications

Specification
ECMAScript Language Specification # sec-asyncgenerator-objects

Browser compatibility

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	Deno	Node.js
`AsyncGenerator`	63	79	55	No	50	12	63	63	55	46	12	8.0	1.0	10.0.0 8.10.0
`next`	63	79	55	No	50	12	63	63	55	46	12	8.0	1.0	10.0.0 8.10.0
`return`	63	79	55	No	50	12	63	63	55	46	12	8.0	1.0	10.0.0 8.10.0
`throw`	63	79	55	No	50	12	63	63	55	46	12	8.0	1.0	10.0.0 8.10.0