Why typed classes matter in Typescript - See It in Action

Why typed classes matter

📖 Scenario: Imagine you are building a simple app to manage books in a library. You want to keep track of each book's title and number of pages. Using typed classes helps you avoid mistakes and makes your code easier to understand.

🎯 Goal: You will create a typed class called Book with properties for title and pages. Then you will create an instance of this class and print its details.

📋 What You'll Learn

Create a class called Book with typed properties title: string and pages: number

Add a constructor to Book that sets title and pages

Create an instance of Book called myBook with title 'The TypeScript Guide' and pages 250

Print the book's title and pages using console.log

💡 Why This Matters

🌍 Real World

Typed classes are used in apps to keep data organized and safe, like managing books, users, or products.

💼 Career

Understanding typed classes is important for writing reliable TypeScript code in many software development jobs.

1

Create the typed class

Create a class called Book with typed properties title: string and pages: number. Add a constructor that takes title: string and pages: number and sets them to the class properties.

Typescript

# Your code here
// Create class Book with typed properties and constructor

Hint

Use class keyword, declare properties with types, and write a constructor method.

2

Create an instance of the class

Create an instance of Book called myBook with title 'The TypeScript Guide' and pages 250.

Typescript

class Book {
  title: string;
  pages: number;

  constructor(title: string, pages: number) {
    this.title = title;
    this.pages = pages;
  }
}

// Create instance myBook here
# Your code here

Hint

Use new Book('The TypeScript Guide', 250) and assign it to myBook.

3

Access and print properties

Use console.log to print the book's title and pages from the myBook instance. Print the title first, then the pages.

Typescript

class Book {
  title: string;
  pages: number;

  constructor(title: string, pages: number) {
    this.title = title;
    this.pages = pages;
  }
}

const myBook = new Book('The TypeScript Guide', 250);

// Print title and pages
# Your code here

Hint

Use console.log(myBook.title) and console.log(myBook.pages).

4

Final output

Run the program to print the book's title and pages exactly as shown: first line The TypeScript Guide, second line 250.

Typescript

class Book {
  title: string;
  pages: number;

  constructor(title: string, pages: number) {
    this.title = title;
    this.pages = pages;
  }
}

const myBook = new Book('The TypeScript Guide', 250);

console.log(myBook.title);
console.log(myBook.pages);

// Run the program to see output

Hint

Make sure you run the program and see the two lines printed exactly.

Practice

1. Why are typed classes important in TypeScript?

easy

A. They make the code run faster at runtime.

B. They allow writing code without any rules.

C. They help catch errors before running the code.

D. They remove the need for functions.

Solution

1. Step 1: Understand typed classes role

   Typed classes define what kind of data each property should hold, helping catch mistakes early.

2. Step 2: Identify the benefit

   This early error detection happens before running the program, improving code safety.

3. Final Answer:

   They help catch errors before running the code. -> Option C

4. Quick Check:

   Typed classes catch errors early = B [OK]

Hint: Typed classes catch mistakes early, not speed up runtime [OK]

Common Mistakes:

• Thinking typed classes speed up code execution
• Believing typed classes remove the need for functions
• Assuming typed classes allow ignoring data rules

2. Which of the following is the correct way to declare a typed class property in TypeScript?

easy

A. class User { name = string; }

B. class User { name: string; }

C. class User { string name; }

D. class User { name -> string; }

Solution

1. Step 1: Recall TypeScript property syntax

   In TypeScript, properties are declared with a colon between name and type, like name: string;.

2. Step 2: Check each option

   class User { name: string; } uses correct syntax. Options A, B, and C use invalid syntax.

3. Final Answer:

   class User { name: string; } -> Option B

4. Quick Check:

   Property declaration uses colon = D [OK]

Hint: Use colon between property and type in classes [OK]

Common Mistakes:

• Using equals sign instead of colon for type
• Placing type before property name
• Using arrows or other symbols instead of colon

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

class Product {
  name: string;
  price: number;
  constructor(name: string, price: number) {
    this.name = name;
    this.price = price;
  }
  display() {
    return `${this.name} costs $${this.price}`;
  }
}
const item = new Product('Book', 15);
console.log(item.display());

medium

A. Book costs $15

B. Book costs $15.00

C. undefined costs $15

D. Error: price is not a string

Solution

1. Step 1: Understand class and constructor

   The constructor sets name to 'Book' and price to 15.

2. Step 2: Check display method output

   The display method returns a string combining name and price exactly as "Book costs $15".

3. Final Answer:

   Book costs $15 -> Option A

4. Quick Check:

   Display method returns formatted string = C [OK]

Hint: Check constructor values and string template output [OK]

Common Mistakes:

• Expecting price to show decimals automatically
• Confusing undefined property values
• Thinking type mismatch causes runtime error here

4. Identify the error in this TypeScript class and fix it:

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

medium

A. Change constructor parameter type to number

B. Change property type to string

C. Remove type annotations completely

D. No error, code is correct

Solution

1. Step 1: Compare property and constructor types

   The property age is a number, but constructor expects a string parameter.

2. Step 2: Fix type mismatch

   Change constructor parameter type to number to match property type.

3. Final Answer:

   Change constructor parameter type to number -> Option A

4. Quick Check:

   Property and parameter types must match = A [OK]

Hint: Match constructor parameter types with property types [OK]

Common Mistakes:

• Ignoring type mismatch between property and parameter
• Changing property type unnecessarily
• Assuming no error when types differ

5. You want to create a typed class in TypeScript that holds a list of users with their names and ages. Which approach best ensures type safety and easy maintenance?

hard

A. Use a class with properties typed as strings only.

B. Use a class with any type for properties to allow flexibility.

C. Use plain JavaScript objects without types for faster coding.

D. Use a class with typed properties and an array of user objects typed with an interface.

Solution

1. Step 1: Consider type safety and maintenance

   Typed properties and interfaces help catch errors and make code clear for teamwork.

2. Step 2: Evaluate options

   A class with typed properties and an array of user objects typed with an interface ensures safety and clarity. Other options sacrifice safety or clarity.

3. Final Answer:

   Use a class with typed properties and an array of user objects typed with an interface. -> Option D

4. Quick Check:

   Typed classes + interfaces = best practice [OK]

Hint: Combine typed classes with interfaces for clear, safe code [OK]

Common Mistakes:

• Using any type loses type safety
• Ignoring interfaces reduces code clarity
• Typing all properties as strings limits data accuracy