await
The await
operator is used to wait for a Promise
. It can only be used inside an async function
within regular JavaScript code; however it can be used on its own with JavaScript modules.
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
Top level await
You can use the await
keyword on its own (outside of an async function) within a JavaScript module. This means modules, with child modules that use await
, wait for the child module to execute before they themselves run. All while not blocking other child modules from loading.
Here is an example of a simple module using the Fetch API and specifying await within the export statement
. Any modules that include this will wait for the fetch to resolve before running any code.
// fetch request
const colors = fetch('../data/colors.json')
.then(response => response.json());
export default await colors;