async function expression
The async function
keyword can be used to define
async
functions inside expressions.
You can also define async functions using an async function statement.
Syntax
async function [name]([param1[, param2[, ..., paramN]]]) {
statements
}
As of ES2015, you can also use arrow functions.
Parameters
name
-
The function name. Can be omitted, in which case the function is anonymous. The name is only local to the function body.
paramN
-
The name of an argument to be passed to the function.
statements
-
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 statement
. The main difference between an async function
expression and an async function
statement 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. See also the chapter about functions for more information.
Examples
Simple example
function resolveAfter2Seconds(x) {
return new Promise(resolve => {
setTimeout(() => {
resolve(x);
}, 2000);
});
};
const add = async function(x) { // async function expression assigned to a variable
let a = await resolveAfter2Seconds(20);
let b = await resolveAfter2Seconds(30);
return x + a + b;
};
add(10).then(v => {
console.log(v); // prints 60 after 4 seconds.
});
(async function(x) { // async function expression used as an IIFE
let p_a = resolveAfter2Seconds(20);
let p_b = resolveAfter2Seconds(30);
return x + await p_a + await p_b;
})(10).then(v => {
console.log(v); // prints 60 after 2 seconds.
});
Specifications
Specification |
---|
ECMAScript Language Specification # sec-async-function-definitions |
Browser compatibility
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See also
async function
AsyncFunction
objectawait