Generator.prototype.next()

The next() method 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(value)

Parameters

value

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 (boolean)

Has the value true if the iterator is past the end of the iterated sequence. In this case value optionally specifies the return value of the iterator.

Has the value false if the iterator was able to produce the next value in the sequence. This is equivalent of not specifying the done property altogether.

value

Any JavaScript value returned by the iterator. Can be omitted when done is true.

Examples

Using next()

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

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

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

Using next() with a list

function* getPage(pageSize = 1, list) {
    let output = [];
    let index = 0;

    while (index < list.length) {
        output = [];
        for (let i = index; i < index + pageSize; i++) {
            if (list[i]) {
                output.push(list[i]);
            }
        }

        yield output;
        index += pageSize;
    }
}

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

page.next();                              // Object {value: (3) [1, 2, 3], done: false}
page.next();                              // Object {value: (3) [4, 5, 6], done: false}
page.next();                              // Object {value: (2) [7, 8], done: false}
page.next();                              // Object {value: undefined, done: true}

Sending values to the generator

In this example, next is called with a value.

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

function* gen() {
  while (true) {
    let value = yield null;
    console.log(value);
  }
}

const g = gen();
g.next(1);
// "{ value: null, done: false }"
g.next(2);
// 2
// "{ value: null, done: false }"

Specifications

Specification
ECMAScript Language Specification
# sec-generator.prototype.next

Browser compatibility

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See also