Generator.prototype.next()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2016.

The next() method of Generator instances returns an object with two properties done and value. You can also provide a parameter to the next method to send a value to the generator.

Syntax

js
next()
next(value)

Parameters

value Optional

The value to send to the generator.

The value will be assigned as a result of a yield expression. For example, in variable = yield expression, the value passed to the .next() function will be assigned to variable.

Return value

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.

Exceptions

TypeError

Thrown if the generator is already running.

Examples

Using next()

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

js
function* gen() {
  yield 1;
  yield 2;
  yield 3;
}

const g = gen(); // Generator { }
g.next(); // { value: 1, done: false }
g.next(); // { value: 2, done: false }
g.next(); // { value: 3, done: false }
g.next(); // { value: undefined, done: true }

Using next() with a list

In this example, getPage takes a list and "paginates" it into chunks of size pageSize. Each call to next will yield one such chunk.

js
function* getPage(list, pageSize = 1) {
  for (let index = 0; index < list.length; index += pageSize) {
    yield list.slice(index, index + pageSize);
  }
}

const list = [1, 2, 3, 4, 5, 6, 7, 8];
const page = getPage(list, 3); // Generator { }

page.next(); // { value: [1, 2, 3], done: false }
page.next(); // { value: [4, 5, 6], done: false }
page.next(); // { value: [7, 8], done: false }
page.next(); // { 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
function* gen() {
  while (true) {
    const value = yield;
    console.log(value);
  }
}

const g = gen();
g.next(1); // Returns { value: undefined, done: false }
// No log at this step: the first value sent through `next` is lost
g.next(2); // Returns { value: undefined, done: false }
// Logs 2