What is Referential Transparency?

An expression, in a program, may be replaced by its value (or anything having the same value) without changing the result of the program. This implies that methods should always return the same value for a given argument, without having any other effect.

An expression is called referentially transparent if it can be replaced with its corresponding value (and vice-versa) without changing the program's behavior.

Avoid side effects

pure function - easier to test