await | JavaScript

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

句法

[rv] = await expression;

expression: Promise or any value to wait for.
rv

描述

await expression causes async function execution to pause until a Promise is settled (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 .

若 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 .

An await can split execution flow, allowing the caller of the await 's function to resume execution before the deferred continuation of the await 's function. After the await defers the continuation of its function, if this is the first await executed by the function, immediate execution also continues by returning to the function's caller a pending Promise for the completion of the await 's function and resuming execution of that caller.

范例

Awaiting a promise to be fulfilled

若 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();

Thenable objects

Thenable objects will be fulfilled just the same.

async function f2() {
  const thenable = {
    then: function(resolve, _reject) {
      resolve('resolved!')
    }
  };
  console.log(await thenable); // resolved!
}
f2();

Conversion to promise

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

async function f3() {
  var y = await 20;
  console.log(y); // 20
}
f3();

Promise rejection

若 Promise is rejected, the rejected value is thrown.

async function f4() {
  try {
    var z = await Promise.reject(30);
  } catch(e) {
    console.error(e); // 30
  }
}
f4();

Handling rejected promises

Handle rejected Promise without try block.

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

规范

规范
ECMAScript (ECMA-262) The definition of 'async functions' in that specification.

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`await`	Chrome 55	Edge 14	Firefox 52	IE No	Opera 42	Safari 10.1	WebView Android 55	Chrome Android 55	Firefox Android 52	Opera Android 42	Safari iOS 10.3	Samsung Internet Android 6.0	nodejs 7.6.0 7.6.0 7.0.0 Disabled Disabled From version 7.0.0: this feature is behind the `--harmony` runtime flag.

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