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 += val;
}
console.log(str);
// Expected output: "abc"
Syntax
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
nameOptional-
The function name. Can be omitted, in which case the function is anonymous. The name is only local to the function body.
paramNOptional-
The name of a formal parameter for the function. For the parameters' syntax, see the Functions reference.
statementsOptional-
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:
const x = function* (y) {
yield y * y;
};
Specifications
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-generator-function-definitions |
Browser compatibility
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