Promise

Promise

A promise object is nothing but returns a future of instance weather the promise is completed, rejected or failed!

We can chain the promises one to another using .then, this is known as nesting of promises!

States of Promises : 

Pending → initial state (neither fulfilled nor rejected)

Fulfilled → the promise is successfully completed and given the respective response

Rejected → the operation is failed (we can catch the error using .catch even in nesting          

For example : 

For Reference : 

Promise - JavaScript | MDN

The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value. To learn about the way promises work and how you can use them, we advise you to read Using promises first. A Promise is a proxy for a value not necessarily known when the promise is created.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise