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yield*

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

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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(...args) {
  yield* [1, 2];
  yield* '34';
  yield* args;
}

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

Browser compatibility

Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet Deno Node.js
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
1.0
4.0.0
0.12.0

See also

© 2005–2022 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/yield*