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
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next
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return
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throw
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See also
 function* async function* function* expression Generator Function Async Generator Function Iterators and generators 

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Last modified: Sep 5, 2022, by MDN contributors


    
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