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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 and a function* expression.

Syntax

function* name([param[, param[, ... param]]]) {
   statements
}
name
The function name.
param
The name of an argument to be passed to the function. A function can have up to 255 arguments.
statements
The statements comprising the body of the function.

Description

Generators are functions which can be exited and later re-entered. Their context (variable bindings) will be saved across re-entrances.

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 statement where execution was paused with the argument from next()

A return statement in a generator, when executed, will make the generator done. If a value is returned, it will be passed back as the value. A generator which has returned will not yield any more values.

Examples

Simple example

function* idMaker() {
  var index = 0;
  while (index < 3)
    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); // undefined
// ...

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(yield);
  console.log(yield);
  console.log(yield);
}

var gen = logGenerator();

// the first call of next executes from the start of the function
// until the first yield statement
gen.next(); 
gen.next('pretzel'); // pretzel
gen.next('california'); // california
gen.next('mayonnaise'); // 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 }

Generators are not constructable

function* f() {}
var obj = new f; // throws "TypeError: f is not a constructor"

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'function*' in that specification.
Standard Initial definition.
ECMAScript 2016 (ECMA-262)
The definition of 'function*' in that specification.
Standard Changed that generators should not have [[Construct]] trap and will throw when used with new.
ECMAScript Latest Draft (ECMA-262)
The definition of 'function*' in that specification.
Living Standard  

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 39.0 25 (13.10586) 26.0 (26.0) No support 26 10
yield* (Yes) 25 (13.10586) 27.0 (27.0) No support 26 10
IteratorResult object instead of throwing (Yes) 25 (13.10586) 29.0 (29.0) No support (Yes) ?
Not constructable with new as per ES2016 (Yes) ? 43.0 (43.0) ? ? ?
Trailing comma in parameters ? ? 52.0 (52.0) ? ? ?
Feature Android Android Webview Edge Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support No support (Yes) ? 26.0 (26.0) No support No support 10 39.0
yield* No support (Yes) ? 27.0 (27.0) No support No support 10 (Yes)
IteratorResult object instead of throwing No support ? ? 29.0 (29.0) No support No support ? (Yes)
Not constructable with new as per ES2016 ? ? ? 43.0 (43.0) ? ? ? ?
Trailing comma in parameters ? ? ? 52.0 (52.0) ? ? ? ?

Firefox-specific notes

Generators and iterators in Firefox versions before 26

Older Firefox versions implement an older version of the generators proposal. In the older version, generators were defined using a regular function keyword (without an asterisk) among other differences. See Legacy generator function for further information.

IteratorResult object returned instead of throwing

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 an IteratorResult object like { value: undefined, done: true } (bug 958951).

See also

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