Composition

Introduction

Composition is a specialized form of association that represents a strong “has-a” relationship where one class (the whole/composite) owns objects of another class (the part), and the parts cannot exist independently of the whole.

Real-World Analogy: House and Rooms

A House contains Rooms:

If the house is destroyed, the rooms are destroyed too
A room cannot exist independently without a house
Rooms are not shared across houses
The house owns the rooms

Characteristics

Characteristic	Description
Relationship	”Part-of” (strong ownership)
Lifetime	Part depends on whole (same lifecycle)
Sharing	Parts cannot be shared
Destroy parent	Child objects are destroyed too
UML	Filled diamond (◆)

Code Example

class Engine {
  constructor(type, horsepower) {
    this.type = type;
    this.horsepower = horsepower;
    this.isRunning = false;
  }
  start() { this.isRunning = true; }
  stop() { this.isRunning = false; }
}

class Wheel {
  constructor(position) {
    this.position = position;
    this.pressure = 32;
  }
}

class Car {
  constructor(make, model) {
    this.make = make;
    this.model = model;
    // Composition: Car creates and owns these components
    this.engine = new Engine("V6", 300);
    this.wheels = [
      new Wheel("Front-Left"),
      new Wheel("Front-Right"),
      new Wheel("Rear-Left"),
      new Wheel("Rear-Right")
    ];
  }

  start() {
    this.engine.start();
    console.log(`${this.make} ${this.model} started`);
  }
}

Composition vs Aggregation

Aspect	Composition	Aggregation
Ownership	Strong (parent owns child)	Weak (parent contains child)
Lifetime	Same lifecycle	Independent lifecycles
Sharing	Not shared	Can be shared
UML	Filled diamond (◆)	Empty diamond (◊)
Child dies with parent?	Yes	No
Example	House has Rooms	Department has Teachers

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