Extending classes with types in Typescript

Introduction

Extending classes with types helps you create new classes based on existing ones. This saves time and keeps your code organized.

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

When you want to reuse code from a base class without rewriting it.

When you want to create a family of related classes with shared behavior.

When you want to add specific properties or methods to a general class.

When you want to keep your code easy to maintain and understand.

Syntax

Typescript

class ChildClass extends ParentClass {
  // new properties or methods
}

The extends keyword means the new class gets all features of the parent class.

You can add new properties or methods inside the child class.

Examples

This example shows a class Dog that extends Animal. It changes the speak method.

Typescript

class Animal {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
  speak() {
    console.log(`${this.name} makes a sound.`);
  }
}

class Dog extends Animal {
  speak() {
    console.log(`${this.name} barks.`);
  }
}

Here, Car extends Vehicle. It uses super to call the parent constructor and adds a new property.

Typescript

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

class Car extends Vehicle {
  brand: string;
  constructor(brand: string) {
    super(4);
    this.brand = brand;
  }
}

Sample Program

This program creates a Person class and a Student class that extends it. The student can greet and study.

Typescript

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

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

const student = new Student("Alice", 10);
student.greet();
student.study();

Important Notes

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

Child classes inherit all public and protected members from the parent class.

You can override methods in the child class to change behavior.

Summary

Extending classes lets you build new classes from existing ones.

Use extends to inherit properties and methods.

Call super() to run the parent constructor inside the child.

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