await

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

Підтримка веб-переглядачами

Див. також

• async function
• async function expression
• AsyncFunction object