1. Asynchronous vs Synchronous

Synchronous

Synchronous execution means the execution happens in a single series. A->B->C->D. If you are calling those routines, A will run, then finish, then B will start, then finish, then C will start, etc.

Asynchronous

With Asynchronous execution, you begin a routine, and let it run in the background while you start your next, then at some point, say "wait for this to finish". It's more like:

Start A->B->C->D->Wait for A to finish

The advantage is that you can execute B, C, and or D while A is still running (in the background, on a separate thread), so you can take better advantage of your resources and have fewer "hangs" or "waits".

2. Example

SYNCHRONOUS

You are in a queue to get a movie ticket. You cannot get one until everybody in front of you gets one, and the same applies to the people queued behind you.

ASYNCHRONOUS

You are in a restaurant with many other people. You order your food. Other people can also order their food, they don't have to wait for your food to be cooked and served to you before they can order. In the kitchen restaurant workers are continuously cooking, serving, and taking orders. People will get their food served as soon as it is cooked.

3. Promises

In JavaScript, a promise is a good way to handle asynchronous operations. It is used to find out if the asynchronous operation is successfully completed or not.

A promise may have one of three states.

1. Pending
2. Fulfilled
3. Rejected

A promise starts in a pending state. That means the process is not complete. If the operation is successful, the process ends in a fulfilled state. And, if an error occurs, the process ends in a rejected state.

3. Async/Await

Async and Await in JavaScript is used to simplify handling asynchronous operations using promises .

In async/await, the async keyword is used to declare an asynchronous function. The await keyword is used to wait for a promise to be resolved before continuing with the execution of the function. The await keyword can only be used inside an async function.

with Async/Await, the await keyword causes the JavaScript engine to pause the execution of the async function until the Promise is resolved or rejected. While the async function waits for the Promise to resolve, it does not block the call stack, and any other synchronous code can be executed. Once the Promise is resolved, the execution of the async function resumes, and the result of the Promise is returned. If rejected, it throws an error value.