Extending classes with types in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to extend the base class correctly.

Typescript

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

class Dog extends [1] {
  bark() {
    return `Woof! I am ${this.name}`;
  }
}

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

AAnimal

BDog

CCat

DPet

Attempts:

3 left

💡 Hint

Common Mistakes

Extending the same class (Dog) instead of the base class (Animal).

Using a class name that does not exist.

The Dog class extends the Animal class to inherit its properties and methods.

2fill in blank

medium

Complete the code to add a type to the extended class property.

Typescript

class Vehicle {
  wheels: number;
  constructor(wheels: number) {
    this.wheels = wheels;
  }
}

class Car extends Vehicle {
  model: [1];
  constructor(wheels: number, model: string) {
    super(wheels);
    this.model = model;
  }
}

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

Anumber

Bstring

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using number or boolean for a text property.

Using any when a specific type is better.

The model property is a string, so its type should be string.

3fill in blank

hard

Fix the error in the constructor to call the parent class correctly.

Typescript

class Person {
  age: number;
  constructor(age: number) {
    this.age = age;
  }
}

class Employee extends Person {
  salary: number;
  constructor(age: number, salary: number) {
    [1];
    this.salary = salary;
  }
}

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

Asuper(age);

Bsuper.salary = salary;

Cthis.age = age;

Dsuper();

Attempts:

3 left

💡 Hint

Common Mistakes

Calling super() without arguments causes an error.

Trying to assign super.salary which is invalid.

The super call must pass the age parameter to the parent constructor.

4fill in blank

hard

Fill both blanks to create a generic class that extends another generic class.

Typescript

class Box<T> {
  content: T;
  constructor(content: T) {
    this.content = content;
  }
}

class ColoredBox<[1]> extends Box<[2]> {
  color: string;
  constructor(content: [2], color: string) {
    super(content);
    this.color = color;
  }
}

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

AT

BU

DV

Attempts:

3 left

💡 Hint

Common Mistakes

Using different generic type parameters in subclass and superclass.

Not specifying generic types in the subclass.

The generic type parameter T is used consistently in both the subclass and the superclass.

5fill in blank

hard

Fill all three blanks to extend an interface with a class and add a new method.

Typescript

interface Logger {
  log(message: string): void;
}

class ConsoleLogger implements [1] {
  [2](message: string): void {
    console.log(message);
  }
  [3](): void {
    console.log('Logging complete');
  }
}

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

ALogger

Blog

Cfinish

DLoggerInterface

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong interface name in implements.

Misspelling the method names.

Not implementing the required interface method.

The class implements the Logger interface, defines the log method, and adds a new finish method.

Practice

1. What does the extends keyword do in TypeScript class definitions?

easy

A. It imports a module into the class.

B. It defines a new interface for a class.

C. It creates a new class that inherits properties and methods from an existing class.

D. It declares a variable inside a class.

Solution

1. Step 1: Understand the role of extends

   The extends keyword is used to create a new class that inherits from an existing class, gaining its properties and methods.

2. Step 2: Differentiate from other keywords

   Other options describe unrelated concepts: interfaces, variables, or modules, which are not what extends does.

3. Final Answer:

   It creates a new class that inherits properties and methods from an existing class. -> Option C

4. Quick Check:

   extends means inheritance [OK]

Hint: Remember: extends means 'is a' relationship in classes [OK]

Common Mistakes:

• Confusing extends with interface implementation
• Thinking extends imports modules
• Using extends to declare variables

2. Which of the following is the correct syntax to extend a class named Animal in TypeScript?

easy

A. class Dog extends Animal {}

B. class Dog inherits Animal {}

C. class Dog implements Animal {}

D. class Dog import Animal {}

Solution

1. Step 1: Recall correct syntax for class extension

   In TypeScript, the keyword to extend a class is extends, so the syntax is class Dog extends Animal {}.

2. Step 2: Identify incorrect options

   inherits is not a TypeScript keyword, implements is for interfaces, and import is for modules.

3. Final Answer:

   class Dog extends Animal {} -> Option A

4. Quick Check:

   Use extends to inherit classes [OK]

Hint: Use 'extends' keyword to inherit classes, not 'inherits' or 'implements' [OK]

Common Mistakes:

• Using 'inherits' instead of 'extends'
• Confusing 'implements' with 'extends'
• Trying to import classes inside class declaration

3. What will be the output of this TypeScript code?

class Person {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
  greet() {
    return `Hello, ${this.name}`;
  }
}

class Student extends Person {
  constructor(name: string) {
    super(name);
  }
}

const s = new Student('Anna');
console.log(s.greet());

medium

A. Hello, undefined

B. Hello, Anna

C. Error: super() not called

D. Hello, Student

Solution

1. Step 1: Understand class inheritance and constructor call

   The Student class extends Person and calls super(name) in its constructor, so the name property is set correctly.

2. Step 2: Check method call and output

   Calling s.greet() uses the inherited method, which returns Hello, Anna.

3. Final Answer:

   Hello, Anna -> Option B

4. Quick Check:

   super() sets parent properties correctly [OK]

Hint: Always call super() in child constructor to set parent properties [OK]

Common Mistakes:

• Forgetting to call super() causes errors
• Expecting child's own greet method without override
• Assuming name is undefined without super()

4. Identify the error in this TypeScript code:

class Vehicle {
  wheels: number;
  constructor(wheels: number) {
    this.wheels = wheels;
  }
}

class Car extends Vehicle {
  constructor() {
    this.wheels = 4;
  }
}

medium

A. Missing call to super() in Car constructor

B. Cannot assign number to wheels property

C. Class Car should not extend Vehicle

D. Constructor syntax is incorrect

Solution

1. Step 1: Check constructor in subclass

   The Car class constructor assigns this.wheels = 4 but does not call super(), which is required before using this.

2. Step 2: Understand TypeScript rules for subclass constructors

   In TypeScript, a subclass constructor must call super() before accessing this. Missing this causes a compile error.

3. Final Answer:

   Missing call to super() in Car constructor -> Option A

4. Quick Check:

   Always call super() before this in subclass constructor [OK]

Hint: Call super() first in subclass constructor before using this [OK]

Common Mistakes:

• Assigning to this before super() call
• Forgetting super() call causes runtime error
• Thinking super() is optional in subclass constructor

5. Given these classes, how can you extend Employee to add a department property and keep the name property from Person?

class Person {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
}

class Employee extends Person {
  // Add department property here
}

hard

A. class Employee extends Person {
department: string;
constructor(name: string, department: string) {
this.department = department;
}
}

B. class Employee extends Person {
department: string;
constructor(department: string) {
this.department = department;
}
}

C. class Employee extends Person {
department: string;
constructor(name: string) {
super(name);
}
}

D. class Employee extends Person {
department: string;
constructor(name: string, department: string) {
super(name);
this.department = department;
}
}

Solution

1. Step 1: Add department property and constructor parameters

   To add department, declare it and add it as a parameter in the constructor along with name.

2. Step 2: Call super(name) and assign department

   Call super(name) to set the inherited name property, then assign this.department = department.

3. Final Answer:

   class Employee extends Person { department: string; constructor(name: string, department: string) { super(name); this.department = department; } } -> Option D

4. Quick Check:

   Use super() for parent props and assign new props in constructor [OK]

Hint: Call super(name) then assign new properties in constructor [OK]

Common Mistakes:

• Not calling super() in constructor
• Missing department parameter in constructor
• Assigning this.department before super() call