function expression

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

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

{{InteractiveExample("JavaScript Demo: function* expression", "taller")}}

```js interactive-example 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

```js-nolint
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_inline}}
• : The function name. Can be omitted, in which case the function is anonymous. The name is only local to the function body.
• paramN {{optional_inline}}
• : The name of a formal parameter for the function. For the parameters' syntax, see the Functions reference.
• statements {{optional_inline}}
• : 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#

{{Specifications}}

Browser compatibility#

{{Compat}}

See also#

• Functions guide
• Functions
• {{jsxref("Statements/function", "function")}}
• {{jsxref("GeneratorFunction")}}
• Iteration protocols
• {{jsxref("Operators/yield", "yield")}}
• {{jsxref("Operators/yield", "yield")}}