A lot of blogs, books, online tutorials, videos, and courses found on the World Wide Web start with a common statement, saying that Redux was built on the top of functional programming. The statement is valid, which means that developers like us need to understand the concept of functional programming.

Let's point out some of the important characteristics of functional programming, as follows:

1. Functions are first class objects
2. Functions can be chained together
3. Functions can be passed as arguments
4. Functions, recursions, and an array can be used to control the flow
5. We can use pure, higher-order, closure, and anonymous functions
6. We can utilize several helper functions, including map, filter, and reduce

In functional programming, functions are considered first class citizens. This means that the language does support passing functions to other functions as arguments, and returning them as the values for other functions. Moreover, they can also be assigned to other variables, or stored in some data structure.