await

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Оператор await використовується для очікування Promise. Він може бути використаний лише в середині async function.

Синтаксис

[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.

Опис

The await expression causes async function execution to pause until  a Promise is fulfilled, that is resolved 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.

Приклади

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

Специфікації

Специфікація Статус Коментар
ECMAScript Latest Draft (ECMA-262)
The definition of 'async functions' in that specification.
Draft Initial definition in ES2017.

Підтримка веб-переглядачами

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
awaitChrome Full support 55Edge Full support 14Firefox Full support 52IE No support NoOpera Full support 42Safari Full support 10.1WebView Android Full support 55Chrome Android Full support 55Firefox Android Full support 52Opera Android Full support 42Safari iOS Full support 10.3Samsung Internet Android Full support 6.0nodejs Full support 7.6.0
Full support 7.6.0
Full support 7.0.0
Disabled
Disabled From version 7.0.0: this feature is behind the --harmony runtime flag.

Legend

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