Promise in Angular

Reason why Both requires?

Because JavaScript is single-threaded, its code executes line by line. If an application needs to wait for something, like a file, web results, or another activity, it waiting until the previous operation is complete is inefficient and a major point of failure.

 

We can avoid this with asynchronous behaviour. There are three JavaScript options for such situations.
Function Callbacks
Promise Items
Asynchronous Functions