The yield
keyword is used to pause and resume a generator function (function*
or legacy generator function).
Syntax
[rv] = yield [expression];
expression
- Defines the value to return from the generator function via the iterator protocol. If omitted,
undefined
is returned instead. rv
-
Returns the optional value passed to the generator's
next()
method to resume its execution.
Description
The yield
keyword causes generator function execution to pause and the value of the expression following the yield
keyword is returned to the generator's caller. It can be thought of as a generator-based version of the return
keyword.
The yield
keyword actually returns an IteratorResult
object with two properties, value
and done
. The value
property is the result of evaluating the yield
expression, and done
is false
, indicating that the generator function has not fully completed.
Once paused on a yield
expression, the generator's code execution remains paused until the generator's next()
method is called. Each time the generator's next()
method is called, the generator resumes execution and runs until it reaches one of the following:
- A
yield
, which causes the generator to once again pause and return the generator's new value. The next timenext()
is called, execution resumes with the statement immediately after theyield
. throw
is used to throw an exception from the generator. This halts execution of the generator entirely, and execution resumes in the caller as is normally the case when an exception is thrown.- The end of the generator function is reached; in this case, execution of the generator ends and an
IteratorResult
is returned to the caller in which thevalue
isundefined
anddone
istrue
. - A
return
statement is reached. In this case, execution of the generator ends and anIteratorResult
is returned to the caller in which thevalue
is the value specified by thereturn
statement anddone
istrue
.
If an optional value is passed to the generator's next()
method, that value becomes the value returned by the generator's current yield
operation.
Between the generator's code path, its yield
operators, and the ability to specify a new starting value by passing it to Generator.prototype.next()
, generators offer enormous power and control.
Examples
The following code is the declaration of an example generator function.
function* foo(){ var index = 0; while (index <= 2) yield index++; }
Once a generator function is defined, it can be used by constructing an iterator as shown.
var iterator = foo(); console.log(iterator.next()); // { value: 0, done: false } console.log(iterator.next()); // { value: 1, done: false } console.log(iterator.next()); // { value: 2, done: false } console.log(iterator.next()); // { value: undefined, done: true }
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
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari (WebKit) |
---|---|---|---|---|---|
Basic support | 39 | 26.0 (26.0) | ? | ? | ? |
IteratorResult object instead of throwing |
? | 29.0 (29.0) | ? | ? | ? |
Feature | Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|
Basic support | (Yes) | 26.0 (26.0) | ? | ? | ? |
IteratorResult object instead of throwing |
? | 29.0 (29.0) | ? | ? | ? |
Firefox-specific notes
- Starting with Gecko 29 (Firefox 29 / Thunderbird 29 / SeaMonkey 2.26), the completed generator function no longer throws a
TypeError
"generator has already finished". Instead, it returns anIteratorResult
object like{ value: undefined, done: true }
(bug 958951). - Starting with Gecko 33 (Firefox 33 / Thunderbird 33 / SeaMonkey 2.30), the parsing of the
yield
expression has been updated to conform with the latest ES6 specification (bug 981599):- The expression after the
yield
keyword is optional and omitting it no longer throws aSyntaxError
:function* foo() { yield; }
- The expression after the