Functional Programming in JavaScript

Pure Functions

A pure function always returns the same output for the same input and produces no side effects. This makes them predictable, testable, and easy to compose. Most of your utility functions should be pure.

Immutability

Avoid mutating data. Use spread operators, Object.freeze, or libraries like Immer to work with immutable data structures. Immutability eliminates an entire class of bugs related to shared mutable state.

Higher-Order Functions

const pipe = (...fns) => (x) =>
  fns.reduce((acc, fn) => fn(acc), x);

const processUser = pipe(
  validateEmail,
  normalizeNames,
  addTimestamps
);

When Not to Go Full Functional

Pragmatism beats purity. Use functional patterns where they add clarity, but don't force them everywhere. Imperative code is sometimes clearer for complex stateful operations. The goal is readable, maintainable code — not adherence to a paradigm.

Practical Applications

• Array methods: map, filter, reduce for data transformations
• Currying for creating reusable, configurable functions
• Option/Result patterns for handling nullable values
• Event sourcing patterns in state management