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:

value

Any JavaScript value yielded or returned by the generator.

Examples

undefined

Using next()

The following example shows a 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

See also