Functional Programming in JavaScript

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What is a Pure Function?

The core of Functional Programming (FP) is the Pure Function. It has two strict rules: 1. Given the same input, it must always return the exact same output. 2. It must have no "Side Effects" (it cannot modify global variables, update the DOM, or mutate the input array). This makes pure functions infinitely testable and perfectly predictable.

Immutability is King

In OOP, you mutate state (user.age = 30). In FP, data is immutable. You never change an object; you create a brand new copy with the updated value. This is why JavaScript introduced the Spread Operator (...) and why methods like .map(), .filter(), and .reduce() are so powerful—they return entirely new arrays without touching the original.

Declarative vs Imperative

Imperative programming focuses on how to do something (writing a manual `for` loop, tracking an index variable). Declarative programming focuses on what you want to do. Using users.filter(u => u.isActive) is declarative. You tell the computer the result you want, and hide the messy implementation details, resulting in code that reads like plain English.