Async and Await are the special syntaxes used to work with promises in Javascript in a comfortable manner.

Both the keywords Async and Await are used with promises in Javascript many times.

Async

Async is written before the function when used and it has a special functionality that when Async will be used it will always return a promise. All the other values will be wrapped with resolved automatically when async is used with a function.

The code example of the Async function is mentioned below -

async function func() { return 1; }

Async can be used with other methods too. However, the code example of async with .then() is mentioned below -

async function func() { return Promise.resolve(1); } func().then(alert);
Output: The promise will be fulfilled and 1 will be returned as an output in the alert window.

Await

When we use Await with the promise it will keep the Javascript waiting until the Promise settles and returns the result.

The syntax of the await is mentioned below -

let promise = await Promise;

The code with execution using the await with promises is mentioned below -

async function func() { let promise = new Promise((resolve, reject) => { setTimeout(() => resolve("Function Executed..!"), 2000) }); let result = await promise; alert(result); } func();
//Output: Function Executed..! will be printed after 2 seconds.

Await will not work if the function is non-async. For instance,

function func() { let pro = Promise.resolve(1); let result = await promise; }
//Output: Syntax Error

There will be a syntax error because the function is non-async. However, this can be resolved using the async keyword before the function.

Error Handling

The Error Handling is the concept that occurs when a await promise raises the error. It throws the error mentioned below -

async function func() { throw new Error("Error"); }

The throw keyword is used here to be used for throwing an error. This can also be done using try and catch as mentioned below -

async function func() { try { let response = await fetch('https://www.google.com'); } catch(err) { alert(err); } } func();