Async/Await

With promises, one can use a async keyword to declare an asynchronous function that returns a promise whereas the await syntax makes JavaScript wait until that promise settles and returns its value. These keywords make promises easier to write. An example of async is shown below.

//async function f
async function f() {
  return 1;
}
// promise being resolved
f().then(alert); // 1

The above example can be written as follows:

function f() {
  return Promise.resolve(1);
}

f().then(alert); // 1

async ensures that the function returns a promise, and wraps non-promises in it. With await, we can make JavaScript wait until the promise is settled with its value returned.

async function f() {
  let promise = new Promise((resolve, reject) => {
    setTimeout(() => resolve("Welcome to Learn JavaScript!"), 1000)
  });
  
  let result = await promise; // wait until the promise resolves (*)
  alert(result); // "Welcome to Learn JavaScript!"
}

f();

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