Hari 15: Functional Programming — Pure Functions & Immutability

Pure Functions

// Pure function: sama input → sama output, tanpa side effects
const tambah = (a, b) => a + b;          // ✅ pure
const kuadrat = x => x * x;              // ✅ pure

// Impure: bergantung state luar
let total = 0;
const tambahTotal = x => { total += x; }; // ❌ side effect

// Impure: random
const acak = () => Math.random();         // ❌ tidak deterministik

Immutability

// Jangan mutasi langsung
const arr = [1, 2, 3, 4, 5];
// arr.push(6); // mutasi — hindari

// Buat array baru
const arrBaru = [...arr, 6];           // [1,2,3,4,5,6]
const tanpaIndeks2 = arr.filter((_, i) => i !== 2); // [1,2,4,5]
const updated = arr.map((n, i) => i === 1 ? 99 : n); // [1,99,3,4,5]

// Object immutable
const user = { nama: "Budi", umur: 25 };
const updatedUser = { ...user, umur: 26 }; // bukan mutasi!

Composition

const pipe = (...fns) => x => fns.reduce((v, f) => f(v), x);
const compose = (...fns) => x => fns.reduceRight((v, f) => f(v), x);

const prosesNama = pipe(
    s => s.trim(),
    s => s.toLowerCase(),
    s => s.replace(/\s+/g, "-")
);
console.log(prosesNama("  Belajar JavaScript  ")); // "belajar-javascript"