Strategy Pattern memungkinkan kamu memilih algoritma atau behavior saat runtime. Alih-alih hardcode satu cara, kamu menyediakan beberapa "strategi" yang bisa ditukar.

Analogi: Navigasi GPS. Kamu pilih strategi: rute tercepat, rute terdekat, atau hindari tol. Tujuan sama, cara berbeda.

Masalah yang diselesaikan:

• Kode penuh if/else untuk memilih perilaku berbeda
• Menambah perilaku baru memerlukan mengubah kode yang sudah ada
• Logika yang seharusnya terpisah jadi tercampur

Tanpa Strategy (kode berantakan):

// BURUK — if/else bertumpuk
function calculatePrice(price, customerType) {
  if (customerType === "regular") {
    return price;
  } else if (customerType === "premium") {
    return price * 0.9;
  } else if (customerType === "vip") {
    return price * 0.8;
  } else if (customerType === "employee") {
    return price * 0.7;
  }
  // Setiap tipe baru = tambah else if...
}

Dengan Strategy Pattern:

// Setiap strategi adalah fungsi terpisah
const pricingStrategies = {
  regular: (price) => price,
  premium: (price) => price * 0.9,
  vip: (price) => price * 0.8,
  employee: (price) => price * 0.7,
};

function calculatePrice(price, customerType) {
  const strategy = pricingStrategies[customerType];
  if (!strategy) throw new Error(`Unknown type: ${customerType}`);
  return strategy(price);
}

console.log(calculatePrice(100000, "vip")); // 80000

// Menambah strategi baru? Cukup tambah 1 baris:
pricingStrategies.student = (price) => price * 0.85;

Contoh: Validasi form

const validators = {
  required: (value) => value.trim() !== "" || "Wajib diisi",
  email: (value) => /^[^@]+@[^@]+$/.test(value) || "Email tidak valid",
  minLength: (min) => (value) =>
    value.length >= min || `Minimal ${min} karakter`,
  maxLength: (max) => (value) =>
    value.length <= max || `Maksimal ${max} karakter`,
};

function validate(value, rules) {
  const errors = [];
  for (const rule of rules) {
    const result = rule(value);
    if (result !== true) errors.push(result);
  }
  return errors;
}

const emailRules = [
  validators.required,
  validators.email,
  validators.minLength(5),
];

console.log(validate("", emailRules));
// ["Wajib diisi", "Email tidak valid", "Minimal 5 karakter"]

console.log(validate("[email protected]", emailRules));
// [] — valid!

Contoh: Sorting strategy

const sortStrategies = {
  byName: (a, b) => a.name.localeCompare(b.name),
  byPrice: (a, b) => a.price - b.price,
  byPriceDesc: (a, b) => b.price - a.price,
  byRating: (a, b) => b.rating - a.rating,
};

function sortProducts(products, strategyName) {
  return [...products].sort(sortStrategies[strategyName]);
}

const products = [
  { name: "Laptop", price: 15000000, rating: 4.5 },
  { name: "Mouse", price: 250000, rating: 4.8 },
  { name: "Keyboard", price: 750000, rating: 4.2 },
];

sortProducts(products, "byPrice");
// Mouse → Keyboard → Laptop

Di JavaScript, Strategy Pattern sangat natural karena fungsi adalah first-class citizen. Kamu tinggal simpan fungsi di object atau array, lalu panggil sesuai kebutuhan.

Kapan pakai Strategy Pattern:

• Saat ada banyak variasi behavior yang bisa ditukar
• Saat if/else atau switch mulai membengkak
• Saat ingin menambah behavior baru tanpa mengubah kode yang ada
• Form validation, sorting, pricing, formatting, authentication