Inheritance using classes in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a class named Animal.

Javascript

class [1] {
  constructor(name) {
    this.name = name;
  }
}

Drag options to blanks, or click blank then click option'

ADog

BBird

CCat

DAnimal

Attempts:

3 left

💡 Hint

Common Mistakes

Using a subclass name like Dog instead of the base class Animal.

The class name should be Animal as it represents the base class.

2fill in blank

medium

Complete the code to make Dog inherit from Animal.

Javascript

class Dog [1] Animal {
  constructor(name, breed) {
    super(name);
    this.breed = breed;
  }
}

Drag options to blanks, or click blank then click option'

Auses

Binherits

Cextends

Dimplements

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'inherits' or 'implements' which are not valid JavaScript keywords for inheritance.

In JavaScript, extends is used to inherit from another class.

3fill in blank

hard

Fix the error in the method overriding speak in Dog class.

Javascript

class Dog extends Animal {
  speak() {
    return [1] + ' barks.';
  }
}

Drag options to blanks, or click blank then click option'

Athis.name

Bname

Csuper.name

DAnimal.name

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'name' without 'this' causes a reference error.

Using 'super.name' which is not valid for property access.

Use this.name to access the instance's name property inside the method.

4fill in blank

hard

Fill both blanks to create a subclass Cat that calls the parent constructor correctly.

Javascript

class Cat [1] Animal {
  constructor([2]) {
    super(name);
  }
}

Drag options to blanks, or click blank then click option'

Aextends

Bname

Cbreed

Dimplements

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'implements' instead of 'extends'.

Passing 'breed' instead of 'name' to super.

The class should extend Animal and the constructor should take name as parameter to pass to super.

5fill in blank

hard

Fill all three blanks to override the speak method in Cat class and call the parent speak method.

Javascript

class Cat extends Animal {
  speak() {
    return super.[1]() + ' and meows.';
  }
  [2]() {
    return this.name;
  }
  constructor([3]) {
    super(name);
  }
}

Drag options to blanks, or click blank then click option'

Aspeak

BgetName

Cname

Dconstructor

Attempts:

3 left

💡 Hint

Common Mistakes

Calling a non-existent parent method.

Using wrong method names or constructor parameters.

The method speak calls super.speak(). The class defines a getName method returning this.name. The constructor takes name as parameter.

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