function*
The function*
declaration (function
keyword followed by an asterisk) defines a generator function, which returns a Generator
object.
You can also define generator functions using the GeneratorFunction
constructor, or the function expression syntax.
Syntax
function* name([param[, param[, ... param]]]) { statements }
name
-
The function name.
-
param
Optional -
The name of a formal parameter for the function.
statements
-
The statements comprising the body of the function.
Description
Generators are functions that can be exited and later re-entered. Their context (variable bindings) will be saved across re-entrances.
Generators in JavaScript -- especially when combined with Promises -- are a very powerful tool for asynchronous programming as they mitigate -- if not entirely eliminate -- the problems with callbacks, such as Callback Hell and Inversion of Control. However, an even simpler solution to these problems can be achieved with async functions.
Calling a generator function does not execute its body immediately; an iterator object for the function is returned instead. When the iterator's next()
method is called, the generator function's body is executed until the first yield
expression, which specifies the value to be returned from the iterator or, with yield*
, delegates to another generator function. The next()
method returns an object with a value
property containing the yielded value and a done
property which indicates whether the generator has yielded its last value, as a boolean. Calling the next()
method with an argument will resume the generator function execution, replacing the yield
expression where an execution was paused with the argument from next()
.
A return
statement in a generator, when executed, will make the generator finish (i.e. the done
property of the object returned by it will be set to true
). If a value is returned, it will be set as the value
property of the object returned by the generator. Much like a return
statement, an error thrown inside the generator will make the generator finished -- unless caught within the generator's body. When a generator is finished, subsequent next()
calls will not execute any of that generator's code, they will just return an object of this form: {value: undefined, done: true}
.
Examples
Simple example
function* idMaker() { var index = 0; while (true) yield index++; } var gen = idMaker(); console.log(gen.next().value); // 0 console.log(gen.next().value); // 1 console.log(gen.next().value); // 2 console.log(gen.next().value); // 3 // ...
Example with yield*
function* anotherGenerator(i) { yield i + 1; yield i + 2; yield i + 3; } function* generator(i) { yield i; yield* anotherGenerator(i); yield i + 10; } var gen = generator(10); console.log(gen.next().value); // 10 console.log(gen.next().value); // 11 console.log(gen.next().value); // 12 console.log(gen.next().value); // 13 console.log(gen.next().value); // 20
Passing arguments into Generators
function* logGenerator() { console.log(0); console.log(1, yield); console.log(2, yield); console.log(3, yield); } var gen = logGenerator(); // the first call of next executes from the start of the function // until the first yield statement gen.next(); // 0 gen.next('pretzel'); // 1 pretzel gen.next('california'); // 2 california gen.next('mayonnaise'); // 3 mayonnaise
Return statement in a generator
function* yieldAndReturn() { yield "Y"; return "R"; yield "unreachable"; } var gen = yieldAndReturn() console.log(gen.next()); // { value: "Y", done: false } console.log(gen.next()); // { value: "R", done: true } console.log(gen.next()); // { value: undefined, done: true }
Generator as an object property
const someObj = { *generator () { yield 'a'; yield 'b'; } } const gen = someObj.generator() console.log(gen.next()); // { value: 'a', done: false } console.log(gen.next()); // { value: 'b', done: false } console.log(gen.next()); // { value: undefined, done: true }
Generator as an object method
class Foo { *generator () { yield 1; yield 2; yield 3; } } const f = new Foo (); const gen = f.generator(); console.log(gen.next()); // { value: 1, done: false } console.log(gen.next()); // { value: 2, done: false } console.log(gen.next()); // { value: 3, done: false } console.log(gen.next()); // { value: undefined, done: true }
Generator as a computed property
class Foo { *[Symbol.iterator] () { yield 1; yield 2; } } const SomeObj = { *[Symbol.iterator] () { yield 'a'; yield 'b'; } } console.log(Array.from(new Foo)); // [ 1, 2 ] console.log(Array.from(SomeObj)); // [ 'a', 'b' ]
Generators are not constructable
function* f() {} var obj = new f; // throws "TypeError: f is not a constructor
Generator defined in an expression
const foo = function* () { yield 10; yield 20; }; const bar = foo(); console.log(bar.next()); // {value: 10, done: false}
Generator example
function* powers(n){ //endless loop to generate for(let current =n;; current *= n){ yield current; } } for(let power of powers(2)){ //controlling generator if(power > 32) break; console.log(power) //2 //4 //8 //16 //32 }
Specifications
Browser compatibility
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | |
function* |
39 |
13 |
26 |
No |
26 |
10 |
39 |
39 |
26 |
26 |
10 |
4.0 |
IteratorResult_object |
49 |
13 |
29 |
No |
36 |
10 |
49 |
49 |
29 |
36 |
10 |
5.0 |
not_constructable_with_new |
50 |
13 |
43 |
No |
37 |
10 |
50 |
50 |
43 |
37 |
10 |
5.0 |
trailing_comma_in_parameters |
58 |
14 |
52 |
No |
45 |
10 |
58 |
58 |
52 |
43 |
10 |
7.0 |
See also
-
function*
expression -
GeneratorFunction
object - Iteration protocols
yield
yield*
-
Function
object -
function
declaration -
function
expression - Functions and function scope
- Other web resources:
- Regenerator an ES2015 generator compiler to ES5
- Forbes Lindesay: Promises and Generators: control flow utopia -- JSConf EU 2013
- Task.js
- Iterating generators asynchronously
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https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function*