Mastering JavaScript Functional Programming - Second Edition

Functions as first-class objects

Saying that functions are first-class objects (also called first-class citizens) means that you can do everything with functions that you can do with other objects. For example, you can store a function in a variable, you can pass it to a function, you can print it out, and so on. This is really the key to doing FP; we will often be passing functions as parameters (to other functions) or returning a function as the result of a function call. 

If you have been doing async Ajax calls, then you have already been using this feature: a callback is a function that will be called after the Ajax call finishes and is passed as a parameter. Using jQuery, you could write something like the following:

$.get("some/url", ...