await

The await operator is used to wait for a Promise returned by an async function.

Syntax

[rv] = await expression;

expression

A Promise or any value to wait for the resolution.

rv

Returns the resolved value of the promise, or the value itself if it's not a Promise.

Description

The await expression causes async function execution to pause, to wait for the Promise's resolution, and to resume the async function execution when the value is resolved. It then returns the resolved value. If the value is not a Promise, it's converted to a resolved Promise.

If the Promise is rejected, the await expression throws the rejected value.

Examples

If a Promise is passed to an await expression, it waits for the Promise's resolution and returns the resolved value.

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

async function f1() {
  var x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}
f1();

If the value is not a Promise, it converts the value to a resolved Promise, and waits for it.

async function f2() {
  var y = await 20;
  console.log(y); // 20
}
f2();

If the Promise is rejected, the rejected value is thrown.

async function f3() {
  try {
    var z = await Promise.reject(30);
  } catch(e) {
    console.log(e); // 30
  }
}
f3();

Specifications

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

Browser compatibility

See also

• async function
• async function expression
• AsyncFunction object