await - JavaScript 编辑

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 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.

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.

An await splits execution flow, allowing the caller of the async function to resume execution. After the await defers the continuation of the async function, execution of subsequent statements ensues. If this await is the last expression executed by its function execution continues by returning to the function's caller a pending Promise for completion of the await's function and resuming execution of that caller.

Examples

Awaiting a promise to be fulfilled

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();

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

If the 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

Specifications

Browser compatibility

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See also

• async function
• async function expression
• AsyncFunction object