Inheritance using classes in Javascript

Introduction

Inheritance lets one class get features from another class. It helps reuse code and organize related things together.

When you want a new object to have all features of an existing object plus some new ones.

When you want to group similar objects but with small differences.

When you want to avoid repeating the same code in many places.

When you want to create a clear relationship between general and specific things.

When you want to extend or customize behavior of existing classes.

Syntax

Javascript

class ParentClass {
  // parent class code
}

class ChildClass extends ParentClass {
  // child class code
}

extends keyword is used to inherit from the parent class.

The child class gets all methods and properties of the parent class automatically.

Examples

Dog inherits from Animal. Dog can use speak() and also has bark().

Javascript

class Animal {
  speak() {
    console.log('Animal speaks');
  }
}

class Dog extends Animal {
  bark() {
    console.log('Dog barks');
  }
}

Car inherits Vehicle but replaces start() with its own version.

Javascript

class Vehicle {
  start() {
    console.log('Vehicle started');
  }
}

class Car extends Vehicle {
  start() {
    console.log('Car started');
  }
}

Sample Program

Student inherits from Person. It uses super to call the parent constructor. Student has all Person features plus study().

Javascript

class Person {
  constructor(name) {
    this.name = name;
  }
  greet() {
    console.log(`Hello, my name is ${this.name}`);
  }
}

class Student extends Person {
  constructor(name, grade) {
    super(name);
    this.grade = grade;
  }
  study() {
    console.log(`${this.name} is studying in grade ${this.grade}`);
  }
}

const student1 = new Student('Alice', 5);
student1.greet();
student1.study();

Important Notes

Use super() in the child constructor to call the parent constructor.

Child classes can add new methods or override existing ones from the parent.

Inheritance helps keep code clean and easy to manage.

Summary

Inheritance lets a class get features from another class using extends.

Child classes can add or change behavior from the parent class.

Use super() to call the parent constructor inside the child.

Practice

1.

What does the extends keyword do in JavaScript classes?

easy

A. It makes one class inherit properties and methods from another class.

B. It creates a new instance of a class.

C. It defines a private method inside a class.

D. It deletes a class from memory.

Solution

1. Step 1: Understand the role of extends

   The extends keyword is used to create a child class that inherits from a parent class.

2. Step 2: Identify inheritance behavior

   This means the child class gets all properties and methods of the parent class automatically.

3. Final Answer:

   It makes one class inherit properties and methods from another class. -> Option A

4. Quick Check:

   extends means inheritance [OK]

Hint: Remember: extends means 'inherits from' [OK]

Common Mistakes:

• Confusing extends with creating instances
• Thinking extends deletes classes
• Mixing extends with private methods

2.

Which of the following is the correct syntax to create a child class Dog that inherits from a parent class Animal?

?

easy

A. class Dog extends Animal {}

B. class Dog inherits Animal {}

C. class Dog : Animal {}

D. class Dog Animal extends {}

Solution

1. Step 1: Recall JavaScript inheritance syntax

   JavaScript uses class Child extends Parent {} to inherit.

2. Step 2: Match syntax to options

   Only class Dog extends Animal {} matches the correct syntax: class Dog extends Animal {}.

3. Final Answer:

   class Dog extends Animal {} -> Option A

4. Quick Check:

   Correct syntax uses extends [OK]

Hint: Use 'extends' keyword to inherit classes [OK]

Common Mistakes:

• Using 'inherits' instead of 'extends'
• Using ':' like other languages
• Wrong order of keywords

3.

What will be the output of this code?

class Animal {
  speak() {
    return "Animal sound";
  }
}

class Dog extends Animal {
  speak() {
    return "Bark";
  }
}

const pet = new Dog();
console.log(pet.speak());

medium

A. undefined

B. "Animal sound"

C. "Bark"

D. Error: speak is not a function

Solution

1. Step 1: Understand method overriding in child class

   The Dog class overrides the speak method from Animal to return "Bark".

2. Step 2: Check which method is called

   When pet.speak() is called, it uses the Dog version, so it returns "Bark".

3. Final Answer:

   "Bark" -> Option C

4. Quick Check:

   Child method overrides parent method [OK]

Hint: Child method replaces parent method if same name [OK]

Common Mistakes:

• Expecting parent method output
• Thinking both methods run
• Confusing method names

4.

Find the error in this code and choose the fix:

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

class Dog extends Animal {
  constructor(name) {
    this.name = name;
  }
}

medium

A. Add extends Animal to Dog class.

B. Add super(name); as the first line in Dog's constructor.

C. Change this.name to super.name in Dog's constructor.

D. Add super(name); as the last line in Dog's constructor.

Solution

1. Step 1: Identify constructor error in child class

   In JavaScript, a child class constructor must call super() before using this.

2. Step 2: Fix by calling super(name);

   Adding super(name); as the first line in Dog's constructor calls the parent constructor properly.

3. Final Answer:

   Add super(name); as the first line in Dog's constructor. -> Option B

4. Quick Check:

   Call super() before this in child constructor [OK]

Hint: Always call super() first in child constructor [OK]

Common Mistakes:

• Using this before super()
• Forgetting to call super()
• Trying to access super.name directly

5.

Given these classes, what will console.log(cat.describe()); print?

class Animal {
  constructor(name) {
    this.name = name;
  }
  describe() {
    return `This is an animal named ${this.name}`;
  }
}

class Cat extends Animal {
  constructor(name, color) {
    super(name);
    this.color = color;
  }
  describe() {
    return `${super.describe()} and its color is ${this.color}`;
  }
}

const cat = new Cat("Whiskers", "black");
console.log(cat.describe());

hard

A. "This is an animal named Whiskers"

B. Error: super.describe is not a function

C. "This is a black animal named Whiskers"

D. "This is an animal named Whiskers and its color is black"

Solution

1. Step 1: Understand super method call in child

   The Cat class calls super.describe() to get the parent description.

2. Step 2: Combine parent and child descriptions

   The child adds its color info, so the full string is "This is an animal named Whiskers and its color is black".

3. Final Answer:

   "This is an animal named Whiskers and its color is black" -> Option D

4. Quick Check:

   super.method() adds parent info in child method [OK]

Hint: Use super.method() to extend parent method output [OK]

Common Mistakes:

• Ignoring super.describe() call
• Expecting only child or parent text
• Thinking super.describe is undefined