How to explain callbacks in plain english? How are they different from calling one function from another function?

Question

How to explain callbacks in plain English? How are they different from calling one function from another function taking some context from the calling function? How can thei

Answer 1

In plain english a callback is a promise. Joe, Jane, David and Samantha share a carpool to work. Joe is driving today. Jane, David and Samantha have a couple of options:

1. Check the window every 5 minutes to see if Joe is out
2. Keep doing their thing until Joe rings the door bell.

Option 1: This is more like a polling example where Jane would be stuck in a "loop" checking if Joe is outside. Jane can't do anything else in the mean time.

Option 2: This is the callback example. Jane tells Joe to ring her doorbell when he's outside. She gives him a "function" to ring the door bell. Joe does not need to know how the door bell works or where it is, he just needs to call that function i.e. ring the door bell when he's there.

Callbacks are driven by "events". In this example the "event" is Joe's arrival. In Ajax for example events can be "success" or "failure" of the asynchronous request and each can have the same or different callbacks.

In terms of JavaScript applications and callbacks. We also need to understand "closures" and application context. What "this" refers to can easily confuse JavaScript developers. In this example within each person's "ring_the_door_bell()" method/callback there might be some other methods that each person need to do based on their morning routine ex. "turn_off_the_tv()". We would want "this" to refer to the "Jane" object or the "David" object so that each can setup whatever else they need done before Joe picks them up. This is where setting up the callback with Joe requires parodying the method so that "this" refers to the right object.

Hope that helps!