What is a callback?

Question

Is it a function?
Is it a function being called from the source?
Or, is it a function being returned from the destination?
Or, is it just executing a function at the destination?
Or, is it a value returned from a function passed to the destination?

Answer 1

A callback is the building block of asynchronous processing.

Think of it this way: when you call someone and they don't answer, you leave a message and your phone number. Later on, the person calls you back based on the phone number you left.

A callback works in a similar manner. You ask an API for a long running operation and you provide a method from within your code to be called with the result of the operation. The API does its work and when the result is ready, it calls your callback method.