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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 resolved, 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.
Examples
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
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript Latest Draft (ECMA-262) The definition of 'async functions' in that specification. |
Draft | |
ECMAScript 2018 (ECMA-262) The definition of 'async functions' in that specification. |
Standard | |
ECMAScript 2017 (ECMA-262) The definition of 'async functions' in that specification. |
Standard | Initial definition. |
Browser compatibility
Desktop | Mobile | Server | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Basic support | Chrome Full support 55 | Edge Full support Yes | Firefox Full support 52 | IE ? | Opera Full support 42 | Safari Full support 10.1 | WebView Android Full support 55 | Chrome Android Full support 55 | Edge Mobile Full support Yes | Firefox Android Full support 52 | Opera Android Full support 42 | Safari iOS Full support 10.1 | Samsung Internet Android Full support 6.0 | nodejs
Full support
7.6.0
|
Legend
- Full support
- Full support
- Compatibility unknown
- Compatibility unknown
- User must explicitly enable this feature.
- User must explicitly enable this feature.