Javascript Async/Await

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Async Syntax
Await Syntax
Example
Browser Support

“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();