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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 April 2017.

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

You can also define async functions using the async function declaration or the arrow syntax.

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

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

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 mimic top-level await. See also the chapter about functions for more information.

Examples

Using async function expression

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

// async function expression assigned to a variable
const add = async function (x) {
  const a = await resolveAfter2Seconds(20);
  const b = await resolveAfter2Seconds(30);
  return x + a + b;
};

add(10).then((v) => {
  console.log(v); // prints 60 after 4 seconds.
});

// async function expression used as an IIFE
(async function (x) {
  const p1 = resolveAfter2Seconds(20);
  const p2 = resolveAfter2Seconds(30);
  return x + (await p1) + (await p2);
})(10).then((v) => {
  console.log(v); // prints 60 after 2 seconds.
});

Specifications

Specification
ECMAScript Language Specification # sec-async-function-definitions

Browser compatibility

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	WebView Android	Deno	Node.js
`async_function`	55	15	52	42	10.1	55	52	42	10.3	6.0	55	1.0	7.6.0