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The await operator is used to wait for a Promise. It can only be used inside an async function.

Syntax

[rv] = await expression;
expression
A Promise or any value to wait for.
rv

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

Description

The await expression causes async function execution to pause until a Promise is resolved, that is fulfilled or rejected, and to resume execution of the async function after fulfillment. When resumed, the value of the await expression is that of the fulfilled Promise.

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

If the value of the expression following the await operator is not a Promise, it's converted to a resolved Promise.

Examples

If a Promise is passed to an await expression, it waits for the Promise to be fulfilled and returns the fulfilled 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();

Handle rejected Promise without try block.

var response = await promisedFunction().catch((err) => { console.log(err); });
// response will be undefined if the promise is rejected

Specifications

Specification Status Comment
ECMAScript Latest Draft (ECMA-262)
The definition of 'async functions' in that specification.
Draft  
ECMAScript 2018 (ECMA-262)
The definition of 'async functions' in that specification.
Standard  
ECMAScript 2017 (ECMA-262)
The definition of 'async functions' in that specification.
Standard Initial definition.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support55 Yes52 ?4210.1
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support5555 Yes524210.16.0

1. From version 7.0.0: this feature is behind the --harmony runtime flag.

See also

Dokumenttaggar och bidragare

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