yield*

The yield* expression 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 2017 Draft (ECMA-262) The definition of 'Yield' in that specification.	Draft

Browser compatibility

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
      *[];
    }

See also

• The Iterator protocol
• function*
• function* expression
• yield