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Javascript Async/Await

2 min read

“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:

 
async 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 keyword await before a function makes the function wait for a promise:

 
let value = await promise;

The await keyword can only be used inside an async function.


Example #

Let’s go slowly and learn how to use it.

Basic Syntax #

async function myDisplay() { let myPromise = new Promise(function(myResolve, myReject) { myResolve("I love You !!"); }); document.getElementById("demo").innerHTML = await myPromise; } myDisplay();

Waiting for a Timeout #

async function myDisplay() { let myPromise = new Promise(function(myResolve, myReject) { setTimeout(function() { myResolve("I love You !!"); }, 3000); }); document.getElementById("demo").innerHTML = await myPromise; } myDisplay();

Waiting for a File #

async function getFile() { let myPromise = new Promise(function(myResolve, myReject) { let req = new XMLHttpRequest(); req.open('GET', "mycar.html"); req.onload = function() { if (req.status == 200) {myResolve(req.response);} else {myResolve("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

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