function* expression

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2016.

The function* keyword can be used to define a generator function inside an expression.

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

Try it

const foo = function* () {
  yield "a";
  yield "b";
  yield "c";
};

let str = "";
for (const val of foo()) {
  str = str + val;
}

console.log(str);
// Expected output: "abc"

Syntax

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

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

Note: An expression statement cannot begin with the keyword function to avoid ambiguity with a function* declaration. The function keyword only begins an expression when it appears in a context that cannot accept statements.

Parameters

name Optional

The function name. Can be omitted, in which case the function is anonymous. The name is only local to the function body.

paramN Optional

The name of a formal parameter for the function. For the parameters' syntax, see the Functions reference.

statements Optional

The statements which comprise the body of the function.

Description

A function* expression is very similar to, and has almost the same syntax as, a function* declaration. The main difference between a function* expression and a function* declaration is the function name, which can be omitted in function* expressions to create anonymous functions. A function* expression can be used as an IIFE (Immediately Invoked Function Expression) which runs as soon as it is defined, allowing you to create an ad-hoc iterable iterator object. See also the chapter about functions for more information.

Examples

Using function* expression

The following example defines an unnamed generator function and assigns it to x. The function yields the square of its argument:

js
const x = function* (y) {
  yield y * y;
};

Specifications

Specification
ECMAScript® 2025 Language Specification
# sec-generator-function-definitions

Browser compatibility

Report problems with this compatibility data on GitHub
desktopmobileserver
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
Deno
Node.js
function* expression
Trailing comma in parameters

Legend

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Full support
Full support

See also