W3cubDocs

/JavaScript

AsyncGenerator.prototype.next()

The next() method of AsyncGenerator instances returns the next value in the sequence.

Syntax

js
next()
next(value)

Parameters

value Optional

An optional value used to modify the internal state of the generator. A value passed to the next() method will be received by yield

Return value

A Promise which when resolved returns an Object with two properties:

done

A boolean value:

  • true if the generator is past the end of its control flow. In this case value specifies the return value of the generator (which may be undefined).
  • false if the generator is able to produce more values.
value

Any JavaScript value yielded or returned by the generator.

Examples

Using next()

The following example shows a simple generator and the object that the next method returns:

js
// 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* createAsyncGenerator() {
  yield delayedValue(500, 1);
  yield delayedValue(500, 2);
  yield delayedValue(500, 3);
}

const asyncGen = createAsyncGenerator();
asyncGen.next().then((res) => console.log(res)); // { value: 1, done: false }
asyncGen.next().then((res) => console.log(res)); // { value: 2, done: false }
asyncGen.next().then((res) => console.log(res)); // { value: 3, done: false }
asyncGen.next().then((res) => console.log(res)); // { value: undefined, done: true }

Sending values to the generator

In this example, next is called with a value.

Note: The first call does not log anything, because the generator was not yielding anything initially.

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

async function* createAsyncGenerator() {
  while (true) {
    await sleep(500);
    const value = yield;
    console.log(value);
  }
}

async function main() {
  const asyncGen = createAsyncGenerator();
  // No log at this step: the first value sent through `next` is lost
  console.log(await asyncGen.next(1)); // { value: undefined, done: false }
  // Logs 2: the value sent through `next`
  console.log(await asyncGen.next(2)); // { value: undefined, done: false }
}

main();

Specifications

Browser compatibility

Desktop Mobile Server
Chrome Edge Firefox Opera Safari Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet WebView Android Deno Node.js
next 63 79 55 50 12 63 55 46 12 8.0 63 1.0 10.0.0

See also

© 2005–2023 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncGenerator/next