O — Open/Closed

O — Open/Closed Principle

Classes should be open for extension, closed for modification.

Add new behavior by extending, not by changing existing code.

Real-World Analogy

A power outlet is closed for modification — you don’t rewire it. But it’s open for extension — you plug in new devices.

❌ Bad Example

class PaymentProcessor {
  process(method, amount) {
    if (method === 'credit') {
      this.#processCredit(amount);
    } else if (method === 'debit') {
      this.#processDebit(amount);
    }
    // ❌ Every new payment type = modify this class
  }

  #processCredit(amount) { /* ... */ }
  #processDebit(amount) { /* ... */ }
}

Why it hurts: To add PayPal, you must modify PaymentProcessor. This risks breaking existing payments.

✅ Fixed Example

// Closed for modification
class PaymentProcessor {
  process(paymentMethod, amount) {
    paymentMethod.pay(amount);
  }
}

// Open for extension — just add new classes
class CreditCard {
  pay(amount) {
    console.log(`Paid $${amount} with Credit Card`);
  }
}

class DebitCard {
  pay(amount) {
    console.log(`Paid $${amount} with Debit Card`);
  }
}

class PayPal {
  pay(amount) {
    console.log(`Paid $${amount} with PayPal`);
  }
}

// Usage
const processor = new PaymentProcessor();
processor.process(new PayPal(), 100); // "Paid $100 with PayPal"

Add a new payment method? Just create a new class. No existing code changes.

How to Spot Violations

switch / if-else chains checking a “type” field
Adding a new feature means editing existing files
“I need to add one more case here…”

When It’s OK to Modify

Bug fixes in existing behavior
Refactoring to improve internal structure (same external behavior)

In Simple Words

Write code that you can add to without changing
Use polymorphism instead of conditionals
New features = new classes, not new if statements
Protects existing, working code from breaking