Class Inheritance

Class Inheritance

Inheritance lets a class inherit properties and methods from another class. The child class gets everything from the parent and can add its own unique features.

The extends Keyword

class User {
    constructor(name, email) {
        this.name = name;
        this.email = email;
    }

    login() {
        console.log(this.name + ' just logged in');
    }

    logout() {
        console.log(this.name + ' just logged out');
    }
}

class Admin extends User {
    deleteUser(user) {
        users = users.filter(function(u) {
            return u.email !== user.email;
        });
        console.log(user.name + ' was deleted by ' + this.name);
    }
}

Using the Subclass

var admin = new Admin('Shaun', 'shaun@example.com');
var user = new User('Mario', 'mario@example.com');

// Admin inherits User methods:
admin.login();   // 'Shaun just logged in'  (inherited)
admin.logout();  // 'Shaun just logged out' (inherited)

// Admin has its own method:
admin.deleteUser(user);  // 'Mario was deleted by Shaun'

super() — Calling the Parent Constructor

class Admin extends User {
    constructor(name, email, role) {
        super(name, email);  // Call the parent constructor
        this.role = role;    // Add new property
    }

    deleteUser(user) {
        console.log(user.name + ' deleted by ' + this.role + ' ' + this.name);
    }
}

var admin = new Admin('Shaun', 'shaun@example.com', 'super-admin');
console.log(admin.name);  // 'Shaun' (from parent)
console.log(admin.role);  // 'super-admin' (from Admin)

super() calls the parent class constructor. You must call it before using this in the child constructor.

Inheritance Hierarchy

User (parent)
  ├── name, email
  ├── login(), logout()
  │
  └── Admin (child) extends User
        ├── Inherits: name, email, login(), logout()
        ├── Own: role
        └── Own: deleteUser()

Key Takeaways

• extends creates a child class that inherits from a parent class
• The child gets all parent properties and methods
• super() calls the parent constructor — required if the child has its own constructor
• Inheritance avoids code duplication between related classes