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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;
}

var 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);
}

var 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';
}

var result;

function* g5() {
  result = yield* g4();
}

var 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}
console.log(iterator.next()); // {value: undefined, done: true}, 
                              // g4() returned {value: 'foo', done: true} at this point

console.log(result);          // "foo"

Specifications

Specification	Status	Comment
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Yield' in that specification.	Standard	Initial definition.
ECMAScript Latest Draft (ECMA-262) The definition of 'Yield' in that specification.	Draft

Browser compatibility

Feature	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari
Basic support	Yes	Yes	27¹	No	Yes	10

Feature	Android webview	Chrome for Android	Edge mobile	Firefox for Android	Opera Android	iOS Safari	Samsung Internet
Basic support	Yes	Yes	Yes	27¹	Yes	10	?

1. Starting with Firefox 33, the parsing of the yield expression has been updated to conform with the ES2015 specification.

Firefox-specific notes

Starting with Gecko 33 (Firefox 33 / Thunderbird 33 / SeaMonkey 2.30), the parsing of the yield expression has been updated to conform with the ES2015 specification (bug 981599):
- The line terminator restriction is now implemented. No line terminator between "yield" and "*" is allowed. Code like the following will throw a SyntaxError:
```
function* foo() {
  yield
  *[];
}
```