What is functional programming?

Functional programming is a programming paradigm — a style of building the structure and elements of computer programs — that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data — Wikipedia

The first fundamental concept we learn when we want to understand functional programming is pure functions.

Here is a very strict definition of purity:

It returns the same result if given the same arguments (it is also referred as deterministic) It does not cause any observable side effects.

Reading Files

If our function reads external files, it’s not a pure function — the file’s contents can change.

Random number generation

Any function that relies on a random number generator cannot be pure.

It does not cause any observable side effects

Examples of observable side effects include modifying a global object or a parameter passed by reference.