In functional programming, everything is seen as a mathematical function, which means for a given input there is a given output and there can be no side effects. Changing a variable's value is considered a side effect and is thus not possible in pure functional programming. To work around this, you typically see a lot of recursive and higher order functions.

Declaring all values as const values is something you would do if you're a diehard functional programmer, as you won't mutate any values anyway.