Inheritance using classes in Javascript - Mini Project: Build & Apply

Inheritance using classes

📖 Scenario: Imagine you are creating a simple program to manage vehicles. You want to keep common features in a general vehicle class and add specific features for cars.

🎯 Goal: You will build two classes: a Vehicle class and a Car class that inherits from Vehicle. You will then create a car object and display its details.

📋 What You'll Learn

Create a Vehicle class with a constructor that takes make and year.

Create a Car class that inherits from Vehicle and adds a model property.

Create an instance of Car with specific values for make, year, and model.

Print the car's details using a method.

💡 Why This Matters

🌍 Real World

Inheritance helps organize code by sharing common features in a base class and adding specific features in child classes. This is useful in many programs like games, business apps, and simulations.

💼 Career

Understanding inheritance is important for software development jobs because it helps write clean, reusable, and organized code.

1

Create the Vehicle class

Create a class called Vehicle with a constructor that takes make and year as parameters and assigns them to this.make and this.year.

Javascript

// Your code here

Hint

Use class keyword to create the class and constructor method to set properties.

2

Create the Car class that inherits Vehicle

Create a class called Car that extends Vehicle. Add a constructor that takes make, year, and model. Call super(make, year) and assign model to this.model.

Javascript

class Vehicle {
    constructor(make, year) {
        this.make = make;
        this.year = year;
    }
}

// Your code here

Hint

Use extends to inherit and super() to call the parent constructor.

3

Create a Car object

Create a variable called myCar and assign it a new Car object with make as "Toyota", year as 2020, and model as "Corolla".

Javascript

class Vehicle {
    constructor(make, year) {
        this.make = make;
        this.year = year;
    }
}

class Car extends Vehicle {
    constructor(make, year, model) {
        super(make, year);
        this.model = model;
    }
}

// Your code here

Hint

Use new Car(...) to create the object with the exact values.

4

Print the car details

Add a method called getDetails inside the Car class that returns a string like "Toyota 2020 Corolla" using this.make, this.year, and this.model. Then, print the result of myCar.getDetails().

Javascript

class Vehicle {
    constructor(make, year) {
        this.make = make;
        this.year = year;
    }
}

class Car extends Vehicle {
    constructor(make, year, model) {
        super(make, year);
        this.model = model;
    }
    
    // Your code here
}

const myCar = new Car("Toyota", 2020, "Corolla");

// Your code here

Hint

Use a method that returns a string with all properties and print it with console.log.

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