async function

The async function declaration defines an asynchronous function, which returns an AsyncFunction object.

Syntax

async function name([param[, param[, ... param]]]) {
   statements
}

name

The function name.

param

The name of an argument to be passed to the function.

statements

The statements comprising the body of the function.

Description

When an async function is called, it returns a Promise. When the async function returns a value, the Promise will be resolved with the returned value. When the async function throws an exception or some value, the Promise will be rejected with the thrown value.

An async function can contain an await expression, that pauses the execution of the async function and waits for the passed Promise's resolution, and then resumes the async function's execution and returns the resolved value.

Examples

Simple example

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

async function add1(x) {
  var a = resolveAfter2Seconds(20);
  var b = resolveAfter2Seconds(30);
  return x + await a + await b;
}

add1(10).then(v => {
  console.log(v);  // prints 60 after 2 seconds.
});

async function add2(x) {
  var a = await resolveAfter2Seconds(20);
  var b = await resolveAfter2Seconds(30);
  return x + a + b;
}

add2(10).then(v => {
  console.log(v);  // prints 60 after 4 seconds.
});

Rewriting a promise chain with an async function

An API that returns a Promise will result in a promise chain, and it splits the function into many parts. Consider the following code:

function getProcessedData(url) {
  return downloadData(url) // returns a promise
    .catch(e => {
      return downloadFallbackData(url) // returns a promise
    })
    .then(v => {
      return processDataInWorker(v); // returns a promise
    });
}

it can be rewritten with a single async function as follows:

async function getProcessedData(url) {
  let v;
  try {
    v = await downloadData(url); 
  } catch(e) {
    v = await downloadFallbackData(url);
  }
  return processDataInWorker(v);
}

Note that in the above example, there is no await statement on the return statement, because the return value of an async function is implicitly wrapped in Promise.resolve.

Specifications

Specification	Status	Comment
ECMAScript 2017 Draft (ECMA-262) The definition of 'async function' in that specification.	Draft	Initial definition in ES2017.

Browser compatibility

See also

• async function expression
• AsyncFunction object
• await