yield*

The yield* is used to delegate to another generator or iterable object.

Syntax

yield* expression

expression: The expression which returns an iterable object.

Description

The yield* expression iterates over the operand and yields each value returned by it.

The value of yield* expression itself is the value returned by that iterator when it's closed (i.e., when done is true).

Examples

Delegating to another generator

In following code, values yielded by g1() are returned from next() calls just like those which are yielded by g2().

function* g1() {
  yield 2;
  yield 3;
  yield 4;
}

function* g2() {
  yield 1;
  yield* g1();
  yield 5;
}

const iterator = g2();

console.log(iterator.next()); // {value: 1, done: false}
console.log(iterator.next()); // {value: 2, done: false}
console.log(iterator.next()); // {value: 3, done: false}
console.log(iterator.next()); // {value: 4, done: false}
console.log(iterator.next()); // {value: 5, done: false}
console.log(iterator.next()); // {value: undefined, done: true}

Other Iterable objects

Besides generator objects, yield* can also yield other kinds of iterables (e.g., arrays, strings, or arguments objects).

function* g3() {
  yield* [1, 2];
  yield* '34';
  yield* Array.from(arguments);
}

const iterator = g3(5, 6);

console.log(iterator.next()); // {value: 1, done: false}
console.log(iterator.next()); // {value: 2, done: false}
console.log(iterator.next()); // {value: "3", done: false}
console.log(iterator.next()); // {value: "4", done: false}
console.log(iterator.next()); // {value: 5, done: false}
console.log(iterator.next()); // {value: 6, done: false}
console.log(iterator.next()); // {value: undefined, done: true}

The value of `yield*` expression itself

yield* is an expression, not a statement—so it evaluates to a value.

function* g4() {
  yield* [1, 2, 3];
  return 'foo';
}

function* g5() {
  const g4ReturnValue = yield* g4();
  console.log(g4ReturnValue) // 'foo'
  return g4ReturnValue;
}

const iterator = g5();

console.log(iterator.next()); // {value: 1, done: false}
console.log(iterator.next()); // {value: 2, done: false}
console.log(iterator.next()); // {value: 3, done: false} done is false because g5 generator isn't finished, only g4
console.log(iterator.next()); // {value: 'foo', done: true}

Specifications

Specification
ECMAScript Language Specification (ECMAScript) # sec-generator-function-definitions-runtime-semantics-evaluation

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`yield*`	39	12	27 Starting with Firefox 33, the parsing of the `yield` expression has been updated to conform with the ES2015 specification.	No	26	10	39	39	27 Starting with Firefox 33, the parsing of the `yield` expression has been updated to conform with the ES2015 specification.	26	10	4.0