5.1. First-class functions

In a programming language, functions are first class if they’re just another kind of object—you can create them at runtime and pass them around your code, including using them as arguments to functions or return values from functions.

First-class functions are a core part of the functional-programming style of programming. Functional programming breaks problems into a set of functions. Preferably, these functions should only have inputs and outputs, neither storing internal state nor having side effects.