In computer science, functional programming (FP) is a programming paradigm and a unique style of building the structure and elements of computer programs. This uniqueness helps treat the computation as the evaluation of mathematical functions and avoids changing-state and mutable data. Thus, by using the FP concept, you can learn to code in your own style that ensures the immutability of your data. In other words, FP is about writing pure functions, about removing hidden inputs and outputs as far as we can, so that as much of our code as possible just describes a relationship between inputs and outputs.

This is not a new concept but the Lambda Calculus, which provides the basis of FP, was first introduced in the 1930s. However, in the realm of programming language, the term functional programming refers to a new style of declarative programming paradigm that means programming can be done with the help of control, declarations, or expressions instead of classical statements commonly used in an old programming language, such as C.