Generator.prototype.next()

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

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:

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.

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.

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

Specifications

Browser compatibility

See also

• function*
• Iterators and generators guide