JavaScript - functional programming

What is functional programming?

This is one of the three popular programming paradigms in use today.

Namely

• Procedural programming - Logic is executed in a set of instructions which execute line by line. Example: C language.
• Object oriented programming - Logic is executed as a "is-a" hierarchy of objects and requires object oriented principles to be followed like encapsulation, polymorphism, abstraction and inheritance among others. Example: C++, Java, .Net etc.
• Functional programming - Logic is written as pure functions without side-effects, mutable state or shared data. Example: Haskel, Closure

Is JavaScript a functional programming language? Answer is yes it can be!

How to identify functional programming style?

1. Immutable state, if you see objects or values directly getting updated in your logic, then it may be mutable state. In FP, state is never directly updated.
2. State should not be shared, if you see the state data being shared among different elements of the application, then its not following FP style.
3. Pure functions - no side effect, side-effect can be as simple as a console.log or as high level as an ajax call. Simply put, side effect cause another part of the application to change based on the execution of a function. This deems that function to be impure and hence not following FP style.
4. Functions as first class entities, to be able to pass around functions as if they are the fundamental pieces of your application, so that the above principles can be easily achieved.
5. Referential transparency, is a by-product of the above principles and essentially it means that instead of a pure function which does not mutate or share outside state, can easily be replaced by the actual returned value of that function.