JavaScript Async
"async and await make promises easier to write"
async makes a function return a Promise
await makes a function wait for a Promise
Async Syntax
The keyword async before a function makes the function return a promise:
Example
async function myFunction() {
return "Hello";
}
Is the same as:
function myFunction() {
return Promise.resolve("Hello");
}
Here is how to use the Promise:
myFunction().then(
function(value) { /* code if successful */ },
function(error) { /* code if some error */ }
);
Example
async function myFunction() {
return "Hello";
}
myFunction().then(
function(value) {myDisplayer(value);},
function(error) {myDisplayer(error);}
);
Or simpler, since you expect a normal value (a normal response, not an error):
Example
async function myFunction() {
return "Hello";
}
myFunction().then(
function(value) {myDisplayer(value);}
);
Await Syntax
The await keyword can only be used inside an
async function.
The await keyword makes the function pause the execution
and wait for a resolved promise before it continues:
let value = await promise;
Example
Let's go slowly and learn how to use it.
Basic Syntax
async function myDisplay() {
let myPromise = new Promise(function(resolve, reject) {
resolve("I love You !!");
});
document.getElementById("demo").innerHTML = await myPromise;
}
myDisplay();
The two arguments (resolve and reject) are pre-defined by JavaScript.
We will not create them, but call one of them when the executor function is ready.
Very often we will not need a reject function.
Example without reject
async function myDisplay() {
let myPromise = new Promise(function(resolve) {
resolve("I love You !!");
});
document.getElementById("demo").innerHTML = await myPromise;
}
myDisplay();
Waiting for a Timeout
async function myDisplay() {
let myPromise = new Promise(function(resolve) {
setTimeout(function() {resolve("I love You !!");}, 3000);
});
document.getElementById("demo").innerHTML = await myPromise;
}
myDisplay();
Waiting for a File
async function getFile() {
let myPromise = new Promise(function(resolve) {
let req = new XMLHttpRequest();
req.open('GET', "mycar.html");
req.onload = function() {
if (req.status == 200) {
resolve(req.response);
} else {
resolve("File not Found");
}
};
req.send();
});
document.getElementById("demo").innerHTML = await myPromise;
}
getFile();
Browser Support
ECMAScript 2017 introduced the JavaScript keywords
async and await.
The following table defines the first browser version with full support for both:
| Chrome 55 | Edge 15 | Firefox 52 | Safari 11 | Opera 42 |
| Dec, 2016 | Apr, 2017 | Mar, 2017 | Sep, 2017 | Dec, 2016 |
