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The yield*
expression 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; } 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
Desktop | Mobile | Server | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Basic support | Chrome Full support Yes | Edge Full support Yes | Firefox
Full support
27
| IE No support No | Opera Full support Yes | Safari Full support 10 | WebView Android Full support Yes | Chrome Android Full support Yes | Edge Mobile Full support Yes | Firefox Android
Full support
27
| Opera Android Full support Yes | Safari iOS Full support 10 | Samsung Internet Android Full support Yes | nodejs
Full support
4.0.0
|
Legend
- Full support
- Full support
- No support
- No support
- See implementation notes.
- See implementation notes.
- User must explicitly enable this feature.
- User must explicitly enable this feature.
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 *[]; }
- The line terminator restriction is now implemented. No line terminator between "yield" and "*" is allowed. Code like the following will throw a