Pure function

A pure function is a special kind of function that always returns the same result when you give it the same input. That is, if you give the same number to a pure function twice, the pure function will always give you the same answer. For example, if you give a pure function the number 2, it will always give back the number 4. This makes pure functions really useful because it means that you can always expect the same answer from the function, no matter what.