async 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 January 2020.

The async function* keywords can be used to define an async generator function inside an expression.

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

Try it

async function joinAll(generator) { let str = ""; for await (const val of generator()) { str = str + val; } return str; } joinAll(async function* () { yield await Promise.resolve("a"); yield await Promise.resolve("b"); yield await Promise.resolve("c"); }).then((str) => console.log(str)); // Expected output: "abc"

Syntax

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

Note: An expression statement cannot begin with the keywords async function to avoid ambiguity with an async function* declaration. The async function keywords only begin an expression when they appear 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

An async function* expression is very similar to, and has almost the same syntax as, an async function* declaration. The main difference between an async function* expression and an async function* declaration is the function name, which can be omitted in async function* expressions to create anonymous functions. An async 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 async iterable object. See also the chapter about functions for more information.

Examples

Using async function* expression

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

const x = async function* (y) { yield Promise.resolve(y * y); }; x(6) .next() .then((res) => console.log(res.value)); // 36

Specifications

Specification
ECMAScript® 2026 Language Specification # sec-async-generator-function-definitions