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function*

The function* declaration creates a {{Glossary("binding")}} of a new generator function to a given name. A generator function can be exited and later re-entered, with its context (variable {{Glossary("binding", "bindings")}}) saved across re-entrances.

You can also define generator functions using the function* expression.

{{EmbedInteractiveExample("pages/js/statement-functionasterisk.html")}}

Syntax​

function* name(param0) {
  statements
}
function* name(param0, param1) {
  statements
}
function* name(param0, param1, /* …, */ paramN) {
  statements
}

[!NOTE] Generator functions do not have arrow function counterparts.

Note: function and * are separate tokens, so they can be separated by whitespace or line terminators.

Parameters​

  • name
    • : The function name.
  • param {{optional_inline}}
    • : The name of a formal parameter for the function. For the parameters' syntax, see the Functions reference.
  • statements {{optional_inline}}
    • : The statements comprising the body of the function.

Description​

A function* declaration creates a {{jsxref("GeneratorFunction")}} object. Each time a generator function is called, it returns a new {{jsxref("Generator")}} object, which conforms to the iterator protocol. When the iterator's next() method is called, the generator function's body is executed until the first {{jsxref("Operators/yield", "yield")}} expression, which specifies the value to be returned from the iterator or, with {{jsxref("Operators/yield*", "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().

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 {{jsxref("Statements/async_function", "async functions", "", 1)}}.

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 behave similar to {{jsxref("Statements/function", "function")}} declarations — they are hoisted to the top of their scope and can be called anywhere in their scope, and they can be redeclared only in certain contexts.

Examples​

Simple example​

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*​

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​

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​

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​

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() {}
const 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 (const power of powers(2)) {
  // controlling generator
  if (power > 32) {
    break;
  }
  console.log(power);
  // 2
  // 4
  // 8
  // 16
  // 32
}

Specifications​

Browser compatibility​

See also​