javascript/Functional Programming

Functional programming in JavaScript centers on a small set of disciplines: pure functions (same input always produces the same output, no side effects), immutability (never mutating data in place, always producing new values), and treating functions as data that can be composed together. None of this is enforced by the language — JavaScript isn't a purely functional language — but the patterns are fully supported because functions are first-class values.

The practical payoff is predictability. A pure function is trivially testable and safely reusable, because it never depends on or changes anything outside itself — no shared mutable state to reason about. Composition takes this further: instead of one large function doing many things, you build small single-purpose functions and combine them (often via compose or pipe utilities) so the overall behavior is the sum of clearly understood parts, which is a large part of why array methods like map, filter, and reduce are considered more functional than an equivalent for loop with mutation.