Promise in javascript is one of the commonly asked questions in JS based interviews these days and also a very helpful concept to learn in javascript.

Promise : What is it?

Promise are similar to real life promise. You always make a promise for a event in future.

Suppose you make a promise to attend a friend's upcoming birthday. There are three states for this promise

1. Pending state - Till the birthday event occurs, it will be in pending state.

2. Successful state - The birthday finally comes and you have successfully kept your promise.

3. Rejected state - The birthday happened but you weren't able to attend and keep your promise due to some issue.

Promise : The Code

new Promise( /* executor */ function(resolve, reject) { ... } );

Promises are used for asynchronous operations where a wait period is usually involved like API calls, I/O operations, file handling etc.

let attendBirthday = true

promise1 = new Promise(function(resolve, reject) {
  if (attendBirthday) {
    resolve("Happy Birthday ");
  } else {
    reject("Sorry I wasnt able to attend");
  }
});
console.log(promise1);

The resolve status gives out value as "Happy Birthday" since the promise is resolved successfully. However, in case the promise returns reject state the value "Sorry I wasnt able to attend" would be returned.

Chaining promise would be covered in the next post.