The yield
keyword is used to pause and resume a generator function (function*
or legacy generator function).
Syntax
[rv] = yield [expression]
expression
Optional- Defines the value to return from the generator function via the iterator protocol. If omitted,
undefined
is returned instead. rv
Optional-
Retrieves the optional value passed to the generator's
next()
method to resume its execution.
Description
The yield
keyword pauses generator function execution 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.
yield
can only be called directly from the generator function that contains it. It cannot be called from nested functions or from callbacks.
The yield
keyword causes the call to the generator's next()
method to return 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.
Unfortunately, next()
is asymmetric, but that canβt be helped: It always sends a value to the currently suspended yield
, but returns the operand of the following yield
.
Examples
Using yield
The following code is the declaration of an example generator function.
function* countAppleSales () { let saleList = [3, 7, 5] for (let i = 0; i < saleList.length; i++) { yield saleList[i] } }
Once a generator function is defined, it can be used by constructing an iterator as shown.
let appleStore = countAppleSales() // Generator { } console.log(appleStore.next()) // { value: 3, done: false } console.log(appleStore.next()) // { value: 7, done: false } console.log(appleStore.next()) // { value: 5, done: false } console.log(appleStore.next()) // { value: undefined, done: true }
You can also send a value with next(value) into the generator. 'step' evaluates as a return value in this syntax [rv] = yield [expression]
function* counter(value) { let step; while (true) { step = yield ++value; if (step) { value += step; } } } const generatorFunc = counter(0); console.log(generatorFunc.next().value); // 1 console.log(generatorFunc.next().value); // 2 console.log(generatorFunc.next().value); // 3 console.log(generatorFunc.next(10).value); // 14 console.log(generatorFunc.next().value); // 15 console.log(generatorFunc.next(10).value); // 26
Specifications
Specification |
---|
ECMAScript (ECMA-262) The definition of 'Yield' in that specification. |
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
yield | Chrome Full support 39 | Edge Full support 12 | Firefox
Full support
26
| IE No support No | Opera Full support 26 | Safari Full support 10 | WebView Android Full support 39 | Chrome Android Full support 39 | Firefox Android
Full support
26
| Opera Android Full support 26 | Safari iOS Full support 10 | Samsung Internet Android Full support 4.0 | 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.