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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