Why typed classes matter in Typescript - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this typed class example?

Consider this TypeScript class with typed properties and a method. What will be printed when the code runs?

Typescript

class Person {
  name: string;
  age: number;

  constructor(name: string, age: number) {
    this.name = name;
    this.age = age;
  }

  greet(): string {
    return `Hello, my name is ${this.name} and I am ${this.age} years old.`;
  }
}

const p = new Person("Alice", 30);
console.log(p.greet());

A "Hello, my name is Alice and I am 30 years old."

B SyntaxError: Unexpected token

C TypeError: Cannot read property 'name' of undefined

D "Hello, my name is undefined and I am undefined years old."

💡 Hint

Look at how the constructor assigns values to the typed properties.

The class Person has typed properties name and age. The constructor sets these correctly. The greet method returns a string using these properties. So the output is the greeting with the given name and age.

🧠 Conceptual

intermediate

Why does TypeScript require types in classes?

Which of the following best explains why typed classes are important in TypeScript?

A They make the code run faster at runtime by optimizing memory.

B They automatically generate user interfaces based on class definitions.

C They allow classes to be used only in browsers that support types.

D They help catch errors early by ensuring properties and methods have expected types.

💡 Hint

Think about what types help with before running the code.

Typed classes help developers catch mistakes early by checking that properties and methods are used with the correct types. This reduces bugs and improves code quality.

🔧 Debug

advanced

What error does this typed class code produce?

Examine this TypeScript class code. What error will occur when compiling?

Typescript

class Car {
  model: string;
  year: number;

  constructor(model: string) {
    this.model = model;
  }

  getAge(currentYear: number): number {
    return currentYear - this.year;
  }
}

A Property 'year' has no initializer and is not definitely assigned in the constructor.

B TypeError: Cannot read property 'year' of undefined

C SyntaxError: Unexpected token in constructor

D No error, code compiles successfully

💡 Hint

Check if all typed properties are assigned values in the constructor.

The property year is declared but never assigned a value in the constructor or with a default. TypeScript requires all properties to be initialized or marked optional.

📝 Syntax

advanced

Which option correctly defines a typed class with a readonly property?

Choose the correct TypeScript class definition where id is a readonly number property set in the constructor.

A class User { id: readonly number; constructor(id: number) { this.id = id; } }

B class User { readonly id: number; constructor(id: number) { this.id = id; } }

C class User { readonly id; constructor(id: number) { this.id = id; } }

D class User { id: number readonly; constructor(id: number) { this.id = id; } }

💡 Hint

Remember the correct order of modifiers and types in TypeScript.

The correct syntax places readonly before the property name and type. Option B follows this rule.

🚀 Application

expert

How many properties does this typed class instance have?

Given this TypeScript class and instance, how many own properties does the instance book have?

Typescript

class Book {
  title: string;
  author: string;
  static category: string = "Literature";

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

const book = new Book("1984", "George Orwell");
const count = Object.keys(book).length;
console.log(count);

A 1

B 3

C 2

D 0

💡 Hint

Static properties are not part of instance own properties.

The instance book has two own properties: title and author. The static property category belongs to the class, not the instance.