Strategy Pattern 🎯

Definition: The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

🎯 Intent

Enable selecting an algorithm's behavior at runtime by encapsulating algorithms in separate classes and making them interchangeable.

🤔 Problem

You have multiple ways of performing a task (like sorting, payment processing, or data validation) and you want to:

Switch between them at runtime
Add new algorithms without modifying existing code
Avoid massive if-else or switch statements

For example, an e-commerce app might support multiple payment methods: credit card, PayPal, cryptocurrency, etc. Without the Strategy pattern, you'd end up with a massive switch statement that violates the Open-Closed Principle.

💡 Solution

The Strategy pattern suggests extracting all different algorithms into separate classes called strategies. The original class (context) gets a field for storing a reference to one of the strategies and delegates the work to the linked strategy object.

🏗️ Structure

Context
├── strategy: Strategy
├── setStrategy(strategy: Strategy)
└── executeStrategy()

Strategy (interface)
└── execute()

ConcreteStrategyA implements Strategy
└── execute()

ConcreteStrategyB implements Strategy
└── execute()

ConcreteStrategyC implements Strategy
└── execute()

💻 Code Example

Basic Implementation

// Strategy interface
class Strategy {
  execute(data) {
    throw new Error("execute() method must be implemented");
  }
}

// Concrete Strategies
class QuickSort extends Strategy {
  execute(data) {
    console.log("Sorting using QuickSort algorithm");
    return this.quickSort([...data]);
  }

  quickSort(arr) {
    if (arr.length <= 1) return arr;

    const pivot = arr[Math.floor(arr.length / 2)];
    const left = arr.filter(x => x < pivot);
    const middle = arr.filter(x => x === pivot);
    const right = arr.filter(x => x > pivot);

    return [...this.quickSort(left), ...middle, ...this.quickSort(right)];
  }
}

class BubbleSort extends Strategy {
  execute(data) {
    console.log("Sorting using BubbleSort algorithm");
    const arr = [...data];

    for (let i = 0; i < arr.length - 1; i++) {
      for (let j = 0; j < arr.length - i - 1; j++) {
        if (arr[j] > arr[j + 1]) {
          [arr[j], arr[j + 1]] = [arr[j + 1], arr[j]];
        }
      }
    }
    return arr;
  }
}

class MergeSort extends Strategy {
  execute(data) {
    console.log("Sorting using MergeSort algorithm");
    return this.mergeSort([...data]);
  }

  mergeSort(arr) {
    if (arr.length <= 1) return arr;

    const mid = Math.floor(arr.length / 2);
    const left = this.mergeSort(arr.slice(0, mid));
    const right = this.mergeSort(arr.slice(mid));

    return this.merge(left, right);
  }

  merge(left, right) {
    const result = [];
    let i = 0, j = 0;

    while (i < left.length && j < right.length) {
      if (left[i] <= right[j]) {
        result.push(left[i++]);
      } else {
        result.push(right[j++]);
      }
    }

    return result.concat(left.slice(i), right.slice(j));
  }
}

// Context
class Sorter {
  constructor(strategy) {
    this.strategy = strategy;
  }

  setStrategy(strategy) {
    this.strategy = strategy;
  }

  sort(data) {
    if (!this.strategy) {
      throw new Error("No sorting strategy set");
    }
    return this.strategy.execute(data);
  }
}

// Usage
const data = [64, 34, 25, 12, 22, 11, 90];
const sorter = new Sorter(new QuickSort());

console.log("Original:", data);
console.log("Sorted:", sorter.sort(data));

// Change strategy at runtime
sorter.setStrategy(new BubbleSort());
console.log("Sorted with Bubble Sort:", sorter.sort(data));

🌟 Real-World Examples

1. Payment Processing System

// Payment strategies
class PaymentStrategy {
  pay(amount) {
    throw new Error("pay() method must be implemented");
  }
}

class CreditCardPayment extends PaymentStrategy {
  constructor(cardNumber, expiryDate, cvv) {
    super();
    this.cardNumber = cardNumber;
    this.expiryDate = expiryDate;
    this.cvv = cvv;
  }

  pay(amount) {
    console.log(`💳 Processing $${amount} via Credit Card`);
    console.log(`Card: ****-****-****-${this.cardNumber.slice(-4)}`);

    // Simulate payment processing
    if (this.validateCard()) {
      console.log("✅ Payment successful!");
      return { success: true, transactionId: `CC_${Date.now()}` };
    }
    return { success: false, error: "Invalid card details" };
  }

  validateCard() {
    return this.cardNumber.length === 16 && this.cvv.length === 3;
  }
}

class PayPalPayment extends PaymentStrategy {
  constructor(email, password) {
    super();
    this.email = email;
    this.password = password;
  }

  pay(amount) {
    console.log(`🅿️ Processing $${amount} via PayPal`);
    console.log(`Account: ${this.email}`);

    if (this.authenticate()) {
      console.log("✅ Payment successful!");
      return { success: true, transactionId: `PP_${Date.now()}` };
    }
    return { success: false, error: "Authentication failed" };
  }

  authenticate() {
    return this.email.includes('@') && this.password.length >= 6;
  }
}

class CryptocurrencyPayment extends PaymentStrategy {
  constructor(walletAddress, privateKey, currency) {
    super();
    this.walletAddress = walletAddress;
    this.privateKey = privateKey;
    this.currency = currency;
  }

  pay(amount) {
    console.log(`₿ Processing $${amount} via ${this.currency}`);
    console.log(`Wallet: ${this.walletAddress.slice(0, 6)}...${this.walletAddress.slice(-4)}`);

    if (this.validateWallet()) {
      console.log("✅ Transaction broadcasted to blockchain!");
      return { success: true, transactionId: `${this.currency}_${Date.now()}` };
    }
    return { success: false, error: "Invalid wallet" };
  }

  validateWallet() {
    return this.walletAddress.length >= 26;
  }
}

// Shopping cart context
class ShoppingCart {
  constructor() {
    this.items = [];
    this.paymentStrategy = null;
  }

  addItem(item, price) {
    this.items.push({ item, price });
  }

  setPaymentStrategy(strategy) {
    this.paymentStrategy = strategy;
  }

  getTotal() {
    return this.items.reduce((total, item) => total + item.price, 0);
  }

  checkout() {
    if (!this.paymentStrategy) {
      console.log("❌ Please select a payment method");
      return false;
    }

    const total = this.getTotal();
    console.log("\n🛒 Order Summary:");
    this.items.forEach(item => {
      console.log(`   ${item.item}: $${item.price}`);
    });
    console.log(`   Total: $${total}`);
    console.log("─".repeat(30));

    return this.paymentStrategy.pay(total);
  }
}

// Usage
const cart = new ShoppingCart();
cart.addItem("Laptop", 999.99);
cart.addItem("Mouse", 29.99);

// Try different payment methods
const creditCard = new CreditCardPayment("1234567812345678", "12/25", "123");
const paypal = new PayPalPayment("user@example.com", "password123");
const bitcoin = new CryptocurrencyPayment("1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa", "private_key", "Bitcoin");

cart.setPaymentStrategy(creditCard);
cart.checkout();

cart.setPaymentStrategy(paypal);
cart.checkout();

cart.setPaymentStrategy(bitcoin);
cart.checkout();

2. Data Validation Strategies

class ValidationStrategy {
  validate(data) {
    throw new Error("validate() method must be implemented");
  }
}

class EmailValidation extends ValidationStrategy {
  validate(email) {
    const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
    const isValid = emailRegex.test(email);

    return {
      isValid,
      message: isValid ? "Valid email" : "Invalid email format"
    };
  }
}

class PasswordValidation extends ValidationStrategy {
  constructor(minLength = 8) {
    super();
    this.minLength = minLength;
  }

  validate(password) {
    const hasMinLength = password.length >= this.minLength;
    const hasUpperCase = /[A-Z]/.test(password);
    const hasLowerCase = /[a-z]/.test(password);
    const hasNumbers = /\d/.test(password);
    const hasSpecialChar = /[!@#$%^&*()_+\-=\[\]{};':"\\|,.<>?]/.test(password);

    const isValid = hasMinLength && hasUpperCase && hasLowerCase && hasNumbers && hasSpecialChar;

    const issues = [];
    if (!hasMinLength) issues.push(`minimum ${this.minLength} characters`);
    if (!hasUpperCase) issues.push("uppercase letter");
    if (!hasLowerCase) issues.push("lowercase letter");
    if (!hasNumbers) issues.push("number");
    if (!hasSpecialChar) issues.push("special character");

    return {
      isValid,
      message: isValid ? "Strong password" : `Password must contain: ${issues.join(", ")}`
    };
  }
}

class PhoneValidation extends ValidationStrategy {
  validate(phone) {
    const phoneRegex = /^\+?[\d\s\-\(\)]{10,}$/;
    const isValid = phoneRegex.test(phone);

    return {
      isValid,
      message: isValid ? "Valid phone number" : "Invalid phone number format"
    };
  }
}

class FormValidator {
  constructor() {
    this.strategies = new Map();
  }

  addValidation(field, strategy) {
    this.strategies.set(field, strategy);
  }

  validate(formData) {
    const results = {};

    for (const [field, strategy] of this.strategies) {
      if (formData.hasOwnProperty(field)) {
        results[field] = strategy.validate(formData[field]);
      }
    }

    return results;
  }
}

// Usage
const validator = new FormValidator();
validator.addValidation('email', new EmailValidation());
validator.addValidation('password', new PasswordValidation(10));
validator.addValidation('phone', new PhoneValidation());

const formData = {
  email: "user@example.com",
  password: "Weak123",
  phone: "+1-555-123-4567"
};

const results = validator.validate(formData);
console.log("Validation Results:", results);

3. Compression Strategies

class CompressionStrategy {
  compress(data) {
    throw new Error("compress() method must be implemented");
  }

  decompress(data) {
    throw new Error("decompress() method must be implemented");
  }
}

class ZipCompression extends CompressionStrategy {
  compress(data) {
    console.log("🗜️  Compressing with ZIP algorithm");
    // Simulate ZIP compression
    const compressed = `ZIP_COMPRESSED[${data.length}]`;
    console.log(`Original: ${data.length} bytes → Compressed: ${compressed.length} bytes`);
    return compressed;
  }

  decompress(compressedData) {
    console.log("📤 Decompressing ZIP data");
    return compressedData.replace("ZIP_COMPRESSED[", "").replace("]", "");
  }
}

class GzipCompression extends CompressionStrategy {
  compress(data) {
    console.log("🗜️  Compressing with GZIP algorithm");
    const compressed = `GZIP_COMPRESSED[${data.length}]`;
    console.log(`Original: ${data.length} bytes → Compressed: ${compressed.length} bytes`);
    return compressed;
  }

  decompress(compressedData) {
    console.log("📤 Decompressing GZIP data");
    return compressedData.replace("GZIP_COMPRESSED[", "").replace("]", "");
  }
}

class RarCompression extends CompressionStrategy {
  compress(data) {
    console.log("🗜️  Compressing with RAR algorithm");
    const compressed = `RAR_COMPRESSED[${data.length}]`;
    console.log(`Original: ${data.length} bytes → Compressed: ${compressed.length} bytes`);
    return compressed;
  }

  decompress(compressedData) {
    console.log("📤 Decompressing RAR data");
    return compressedData.replace("RAR_COMPRESSED[", "").replace("]", "");
  }
}

class FileCompressor {
  constructor(strategy) {
    this.strategy = strategy;
  }

  setStrategy(strategy) {
    this.strategy = strategy;
  }

  compressFile(filename, data) {
    console.log(`\n📁 Processing file: ${filename}`);
    return this.strategy.compress(data);
  }

  decompressFile(compressedData) {
    return this.strategy.decompress(compressedData);
  }
}

// Usage
const data = "This is a sample file content that needs to be compressed...";
const compressor = new FileCompressor(new ZipCompression());

// Compress with ZIP
let compressed = compressor.compressFile("document.txt", data);

// Switch to GZIP
compressor.setStrategy(new GzipCompression());
compressed = compressor.compressFile("document.txt", data);

// Switch to RAR
compressor.setStrategy(new RarCompression());
compressed = compressor.compressFile("document.txt", data);

✅ Pros

Open/Closed Principle: You can introduce new strategies without changing existing code
Runtime Algorithm Selection: You can switch algorithms at runtime
Elimination of Conditionals: Replaces large switch statements or if-else chains
Isolation of Implementation Details: Each strategy encapsulates its algorithm
Code Reusability: Strategies can be reused across different contexts

❌ Cons

Increased Number of Classes: Can result in many small classes
Client Awareness: Clients must be aware of different strategies to choose the appropriate one
Communication Overhead: Context and strategies might need to exchange data
Strategy Interface Complexity: If strategies have very different requirements, the interface might become complex

🎯 When to Use

Multiple Algorithm Variants: When you have multiple ways of performing a task
Runtime Selection: When you want to choose algorithms at runtime
Avoiding Conditionals: When you want to eliminate large conditional statements
Plugin Architecture: When building systems that support plugins or extensions
A/B Testing: When you need to test different implementations

🔄 Common Variations

1. Strategy with State

class StatefulStrategy extends Strategy {
  constructor() {
    super();
    this.state = {};
  }

  execute(data) {
    // Use and modify internal state
    this.state.executionCount = (this.state.executionCount || 0) + 1;
    return this.process(data);
  }

  process(data) {
    throw new Error("process() method must be implemented");
  }
}

2. Strategy Factory

class StrategyFactory {
  static createStrategy(type) {
    switch(type) {
      case 'quick': return new QuickSort();
      case 'bubble': return new BubbleSort();
      case 'merge': return new MergeSort();
      default: throw new Error(`Unknown strategy type: ${type}`);
    }
  }
}

// Usage
const strategy = StrategyFactory.createStrategy('quick');
const sorter = new Sorter(strategy);

3. Async Strategies

class AsyncStrategy {
  async execute(data) {
    throw new Error("execute() method must be implemented");
  }
}

class AsyncContext {
  constructor(strategy) {
    this.strategy = strategy;
  }

  async performOperation(data) {
    return await this.strategy.execute(data);
  }
}

State: Both patterns use composition and delegation, but Strategy focuses on algorithms while State focuses on behavior changes
Template Method: Both patterns deal with varying algorithms, but Strategy uses composition while Template Method uses inheritance
Bridge: Similar structure, but Bridge separates interface from implementation while Strategy makes algorithms interchangeable
Factory Method: Often used together to create appropriate strategies

📈 Performance Considerations

Memory Usage: Each strategy creates a separate object
Strategy Switching Cost: Changing strategies has minimal overhead
Algorithm Complexity: Choose strategies based on data size and performance requirements

🎯 Intent​

🤔 Problem​

💡 Solution​

🏗️ Structure​

💻 Code Example​

Basic Implementation​

🌟 Real-World Examples​

1. Payment Processing System​

2. Data Validation Strategies​

3. Compression Strategies​

✅ Pros​

❌ Cons​

🎯 When to Use​

🔄 Common Variations​

1. Strategy with State​

2. Strategy Factory​

3. Async Strategies​

🔗 Related Patterns​

📈 Performance Considerations​

📚 Further Reading​