Class property declarations in Typescript - Mini Project: Build & Apply

Class property declarations

📖 Scenario: You are creating a simple program to represent a book in a library system. Each book has a title, an author, and a number of pages.

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

📋 What You'll Learn

Create a class called Book

Declare three properties inside the class: title (string), author (string), and pages (number)

Create a variable called myBook that is an instance of Book

Assign values to the properties of myBook

Print the book details in the format: Title: [title], Author: [author], Pages: [pages]

💡 Why This Matters

🌍 Real World

Classes with properties are used to model real-world objects in software, like books in a library or products in a store.

💼 Career

Understanding how to declare and use class properties is fundamental for object-oriented programming, which is widely used in software development jobs.

1

Create the Book class with properties

Create a class called Book with three properties: title of type string, author of type string, and pages of type number. Do not add any constructor or methods yet.

Typescript

# Create the Book class with three properties
class Book {
    // Your code here
}

Hint

Use class Book {} and inside declare title: string;, author: string;, and pages: number;.

2

Create an instance of Book

Create a variable called myBook and assign it a new instance of the Book class.

Typescript

class Book {
    title: string;
    author: string;
    pages: number;
}

// Create myBook as a new Book instance
// Your code here

Hint

Use const myBook = new Book(); to create the instance.

3

Assign values to myBook properties

Assign the following values to the properties of myBook: title = "The Great Gatsby", author = "F. Scott Fitzgerald", and pages = 180.

Typescript

class Book {
    title: string;
    author: string;
    pages: number;
}

const myBook = new Book();

// Assign values to myBook properties
// Your code here

Hint

Use dot notation like myBook.title = "The Great Gatsby"; to assign values.

4

Print the book details

Write a console.log statement to print the book details in this exact format: Title: The Great Gatsby, Author: F. Scott Fitzgerald, Pages: 180 using the properties of myBook.

Typescript

class Book {
    title: string;
    author: string;
    pages: number;
}

const myBook = new Book();
myBook.title = "The Great Gatsby";
myBook.author = "F. Scott Fitzgerald";
myBook.pages = 180;

// Print the book details
// Your code here

Hint

Use a template string with backticks and ${} to insert property values inside console.log.

Practice

1. What is the purpose of declaring properties inside a TypeScript class?

easy

A. To import external modules into the class

B. To define functions that the class can use

C. To create new classes inside the current class

D. To store data related to each object created from the class

Solution

1. Step 1: Understand what class properties represent

   Class properties hold data or information specific to each object created from the class.

2. Step 2: Differentiate properties from methods and other elements

   Functions inside classes are called methods, not properties. Properties are for storing values.

3. Final Answer:

   To store data related to each object created from the class -> Option D

4. Quick Check:

   Class properties = store object data [OK]

Hint: Properties hold object data, methods hold actions [OK]

Common Mistakes:

• Confusing properties with methods
• Thinking properties create new classes
• Believing properties import modules

2. Which of the following is the correct way to declare a class property with a type and default value in TypeScript?

easy

A. class Car { speed: = 0 number; }

B. class Car { speed: number = 0; }

C. class Car { number speed = 0; }

D. class Car { speed = number 0; }

Solution

1. Step 1: Recall TypeScript property syntax

   Properties are declared as name: type = defaultValue;.

2. Step 2: Check each option for correct syntax

   class Car { speed: number = 0; } matches the correct syntax: speed: number = 0;. Others have incorrect order or missing punctuation.

3. Final Answer:

   class Car { speed: number = 0; } -> Option B

4. Quick Check:

   Property syntax = name: type = value [OK]

Hint: Use colon for type, equals for default value [OK]

Common Mistakes:

• Swapping type and value order
• Missing colon between name and type
• Using wrong punctuation

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

class Person {
  name: string = "Alice";
  age: number = 30;
}
const p = new Person();
console.log(p.name + ' is ' + p.age + ' years old');

medium

A. "Alice is 30 years old"

B. "undefined is undefined years old"

C. Error: Property 'name' does not exist

D. "Alice is undefined years old"

Solution

1. Step 1: Understand property initialization

   The class Person has properties name and age initialized with "Alice" and 30 respectively.

2. Step 2: Check the output of console.log

   Creating an object p sets these properties. The log concatenates them into the string "Alice is 30 years old".

3. Final Answer:

   "Alice is 30 years old" -> Option A

4. Quick Check:

   Initialized properties print correct values [OK]

Hint: Default values print as expected when object created [OK]

Common Mistakes:

• Assuming properties are undefined without constructor
• Confusing property names or types
• Expecting runtime errors without syntax mistakes

4. Identify the error in this TypeScript class property declaration:

class Book {
  title string = "Unknown";
  pages: number = 100;
}

medium

A. Class name should be lowercase

B. Missing default value for 'pages' property

C. Missing colon ':' between property name and type in 'title string = "Unknown";'

D. Properties cannot have default values

Solution

1. Step 1: Check syntax of property declarations

   Property declarations require a colon ':' between the name and type, like title: string.

2. Step 2: Identify the incorrect line

   The line title string = "Unknown"; misses the colon, causing a syntax error.

3. Final Answer:

   Missing colon ':' between property name and type in 'title string = "Unknown";' -> Option C

4. Quick Check:

   Property syntax requires colon ':' [OK]

Hint: Always put ':' between property name and type [OK]

Common Mistakes:

• Omitting colon between name and type
• Thinking default values are not allowed
• Ignoring case sensitivity of class names (not an error)

5. You want to create a TypeScript class Student with a property grades that stores an array of numbers and defaults to an empty array. Which is the correct declaration to ensure each object has its own separate array?

hard

A. class Student { grades: number[] = []; }

B. class Student { static grades: number[] = []; }

C. class Student { grades: number[]; } // no default value

D. class Student { grades = []; }

Solution

1. Step 1: Understand instance vs static properties

   Instance properties belong to each object separately; static properties are shared by all objects.

2. Step 2: Check which option creates separate arrays per object

   class Student { grades: number[] = []; } declares grades as an instance property with default empty array, so each object gets its own array.

3. Step 3: Analyze other options

   class Student { static grades: number[] = []; } uses static, so all objects share one array (not desired). class Student { grades: number[]; } // no default value has no default, so array is undefined unless set later. class Student { grades = []; } misses type annotation explicitly, inferring any[] instead of number[].

4. Final Answer:

   class Student { grades: number[] = []; } -> Option A

5. Quick Check:

   Instance property with default array = separate arrays [OK]

Hint: Use instance property with default [] for separate arrays [OK]

Common Mistakes:

• Using static for properties needing separate values
• Not initializing arrays causing undefined errors
• Omitting type annotation (less clear)