Why does the principle "Composition over Inheritance" recommend preferring composition over classical inheritance?

Short answer

Inheritance creates tight, fixed compile-time coupling and leads to the fragile base class problem, while composition allows behavior to be swapped dynamically at runtime with greater flexibility.

Explanation

Inheritance represents an "is-a" relationship that exposes parent-class details to the child (white-box reuse), and changing the parent can unintentionally break children (the Fragile Base Class Problem). Composition represents a "has-a" relationship (black-box reuse): an object holds references to other objects implementing specific interfaces, making testing and component replacement easier.

Code example

// Instead of rigid inheritance: class FlyingSwimmingDuck extends Duck
class Duck {
  constructor(private flyBehavior: FlyBehavior, private swimBehavior: SwimBehavior) {}
  performFly() { this.flyBehavior.fly(); }
}

Common mistakes

• Using inheritance just to reuse a few lines of code when there is no real relationship between the types.

Follow-up questions

• What is polymorphism, and what is the difference between static dispatch and dynamic dispatch?

Sources

• Composition over inheritance - Wikipedia