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.
Try it
Syntax
js
function* name(param0) {
statements
}
function* name(param0, param1) {
statements
}
function* name(param0, param1, /* … ,*/ paramN) {
statements
}
Note: Generator functions do not have arrow function counterparts.
Parameters
name
-
The function name.
param
Optional-
The name of a formal parameter for the function.
statements
Optional-
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; a generator
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}
.
function*
declarations are hoisted to the top of their scope and can be called anywhere in their scope.
Examples
Simple example
js
function* idMaker() {
let index = 0;
while (true) {
yield index++;
}
}
const 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*
js
function* anotherGenerator(i) {
yield i + 1;
yield i + 2;
yield i + 3;
}
function* generator(i) {
yield i;
yield* anotherGenerator(i);
yield i + 10;
}
const 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
js
function* logGenerator() {
console.log(0);
console.log(1, yield);
console.log(2, yield);
console.log(3, yield);
}
const 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
js
function* yieldAndReturn() {
yield "Y";
return "R";
yield "unreachable";
}
const 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
js
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
js
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
js
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
js
function* f() {}
const obj = new f(); // throws "TypeError: f is not a constructor
Generator defined in an expression
js
const foo = function* () {
yield 10;
yield 20;
};
const bar = foo();
console.log(bar.next()); // {value: 10, done: false}
Generator example
js
function* powers(n) {
//endless loop to generate
for (let current = n; ; current *= n) {
yield current;
}
}
for (const power of powers(2)) {
// controlling generator
if (power > 32) {
break;
}
console.log(power);
// 2
// 4
// 8
// 16
// 32
}
Specifications
Specification |
---|
ECMAScript Language Specification # sec-generator-function-definitions |
Browser compatibility
BCD tables only load in the browser
See also
function*
expressionGeneratorFunction
object- Iteration protocols
yield
yield*
Function
objectfunction
declarationfunction
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