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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);});


Try it Yourself »


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);}
);


Try it Yourself »



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();


Try it Yourself »



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();


Try it Yourself »



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();


Try it Yourself »



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();


Try it Yourself »




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
















★
+1

Reference: https://www.w3schools.com/js/js_async.asp