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
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.
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
Specification |
---|
ECMAScript (ECMA-262) The definition of 'async functions' in that specification. |
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
await | Chrome Full support 55 | Edge Full support 14 | Firefox Full support 52 | IE No support No | Opera Full support 42 | Safari Full support 10.1 | WebView Android Full support 55 | Chrome Android Full support 55 | Firefox Android Full support 52 | Opera Android Full support 42 | Safari iOS Full support 10.3 | Samsung Internet Android Full support 6.0 | nodejs
Full support
7.6.0
|
Legend
- Full support
- Full support
- No support
- No support
- User must explicitly enable this feature.
- User must explicitly enable this feature.