Association is a relationship between two classes where one class knows about or is connected to another class. It represents a “uses-a” relationship where objects can communicate with each other.

• Models real-world relationships where entities interact with each other
• Enables collaboration between objects
• Forms the basis for more specific relationships (aggregation, composition)

A teacher and a student have an association:

• They know about each other
• They interact (teacher teaches, student learns)
• They have independent lifetimes — a teacher exists without a specific student
• The relationship can be one-way (student knows their teacher) or two-way

One class knows about the other, but not vice versa.

class Order {
  constructor(id, customer) {
    this.id = id;
    this.customer = customer; // Order knows about Customer
  }
}

class Customer {
  constructor(name) { this.name = name; }
}

Both classes know about each other.

class Doctor {
  constructor(name) {
    this.name = name;
    this.patients = [];
  }
  addPatient(patient) {
    this.patients.push(patient);
    patient.addDoctor(this);
  }
}

class Patient {
  constructor(name) {
    this.name = name;
    this.doctors = [];
  }
  addDoctor(doctor) { this.doctors.push(doctor); }
}

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