Class property declarations in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of class property initialization

What is the output of this TypeScript code when creating an instance of Person and logging person.name?

Typescript

class Person {
  name: string = "Alice";
  constructor() {
    this.name = "Bob";
  }
}
const person = new Person();
console.log(person.name);

A "Alice"

B "Bob"

C undefined

D TypeError

💡 Hint

Class property initializers run before the constructor body.

The property name is first initialized to "Alice". Then the constructor runs and sets name to "Bob". So the final value is "Bob".

❓ Predict Output

intermediate

Readonly class property behavior

What happens when this TypeScript code runs?

Typescript

class Car {
  readonly model: string = "Tesla";
  constructor() {
    this.model = "BMW";
  }
}
const car = new Car();
console.log(car.model);

A "Tesla"

B "BMW"

C Compile-time error

D Runtime error

💡 Hint

Readonly properties cannot be assigned after initialization.

The assignment to this.model inside the constructor is not allowed because model is readonly. TypeScript will report a compile-time error.

🔧 Debug

advanced

Why does this code throw an error?

This TypeScript class has a property declared but the code throws an error when creating an instance. What is the cause?

Typescript

class Counter {
  count: number;
  increment() {
    this.count++;
  }
}
const c = new Counter();
c.increment();
console.log(c.count);

A "count" is not initialized before use, causing undefined increment error

B "count" should be declared as readonly

C Class must have a constructor to declare properties

D "increment" method is missing a return type

💡 Hint

Check if the property has a starting value before incrementing.

The property count is declared but not initialized, so it is undefined. Incrementing undefined causes a runtime error.

📝 Syntax

advanced

Identify the syntax error in class property declaration

Which option contains a syntax error in declaring a class property in TypeScript?

Typescript

class User {
  // property declarations here
}

A email string = "[e-mail]";

B age: number;

C isActive = true;

D name: string = "John";

💡 Hint

Check the order of type and variable name.

Option A misses the colon between the property name and its type. It should be email: string = "[e-mail]";

🚀 Application

expert

How many properties does this class instance have?

Given this TypeScript class, how many own properties will an instance have immediately after creation?

Typescript

class Book {
  title: string = "";
  author: string;
  static category: string = "Fiction";
  constructor(author: string) {
    this.author = author;
  }
}
const novel = new Book("Jane Austen");

A 1

B 3

C 0

D 2

💡 Hint

Static properties are not part of instance properties.

The instance novel has two own properties: title (initialized) and author (set in constructor). The static property category belongs to the class, not the instance.