Readonly class properties in Typescript - Mini Project: Build & Apply

Readonly class properties

📖 Scenario: Imagine you are creating a program to manage books in a library. Each book has a title and an author. Once a book is created, its title and author should never change.

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

📋 What You'll Learn

Create a class named Book with readonly properties title and author

Add a constructor to Book that sets title and author

Create an instance of Book with title 'The Great Gatsby' and author 'F. Scott Fitzgerald'

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

💡 Why This Matters

🌍 Real World

Readonly properties are useful when you want to protect important data from being changed after creation, like book details, user IDs, or configuration settings.

💼 Career

Understanding readonly properties helps you write safer and more predictable TypeScript code, which is valuable in many software development jobs.

1

Create the Book class with readonly properties

Create a class called Book with readonly properties title and author of type string.

Typescript

class Book {
  // Your code here
}

Hint

Use the readonly keyword before the property names to make them immutable.

2

Add a constructor to set the readonly properties

Add a constructor to the Book class that takes parameters title and author of type string and assigns them to the readonly properties.

Typescript

class Book {
  readonly title: string;
  readonly author: string;

  // Your code here
}

Hint

The constructor sets the readonly properties when a new Book is created.

3

Create an instance of Book

Create a variable called myBook and assign it a new Book object with title 'The Great Gatsby' and author 'F. Scott Fitzgerald'.

Typescript

class Book {
  readonly title: string;
  readonly author: string;

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

// Your code here

Hint

Use the new keyword to create an instance of the Book class.

4

Print the book's title and author

Use console.log to print the book's title and author from the myBook object in the format: Title: The Great Gatsby, Author: F. Scott Fitzgerald.

Typescript

class Book {
  readonly title: string;
  readonly author: string;

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

const myBook = new Book('The Great Gatsby', 'F. Scott Fitzgerald');

// Your code here

Hint

Use a template string with backticks and ${} to insert the property values.

Practice

1. What does the readonly keyword do when used on a class property in TypeScript?

easy

A. It allows the property to be changed anytime.

B. It makes the property private and inaccessible outside the class.

C. It prevents the property from being changed after initialization.

D. It deletes the property after the constructor runs.

Solution

1. Step 1: Understand the purpose of readonly

   The readonly keyword marks a property so it cannot be changed after it is set initially.

2. Step 2: Check when assignment is allowed

   Assignment to a readonly property is only allowed during declaration or inside the constructor.

3. Final Answer:

   It prevents the property from being changed after initialization. -> Option C

4. Quick Check:

   Readonly means fixed value after set [OK]

Hint: Readonly means value can't change after constructor [OK]

Common Mistakes:

• Thinking readonly makes property private
• Assuming readonly allows changes anytime
• Confusing readonly with deleting properties

2. Which of the following is the correct way to declare a readonly property id of type number in a TypeScript class?

easy

A. id: readonly number;

B. readonly id: number;

C. let readonly id: number;

D. const id: number;

Solution

1. Step 1: Recall TypeScript readonly syntax

   The correct syntax places readonly before the property name and type.

2. Step 2: Check each option

   readonly id: number; matches the correct syntax: readonly id: number;. Others misuse keywords or order.

3. Final Answer:

   readonly id: number; -> Option B

4. Quick Check:

   Readonly keyword before property name [OK]

Hint: Put readonly before property name, not after type [OK]

Common Mistakes:

• Using const inside class properties
• Placing readonly after the type
• Combining let with readonly incorrectly

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

class User {
  readonly name: string;
  constructor(name: string) {
    this.name = name;
  }
  changeName(newName: string) {
    // @ts-ignore
    this.name = newName;
  }
}
const user = new User('Alice');
user.changeName('Bob');
console.log(user.name);

medium

A. Alice

B. undefined

C. Compilation error

D. Bob

Solution

1. Step 1: Understand readonly assignment rules

   The name property is readonly and assigned in the constructor, so initial value is 'Alice'. TypeScript enforces this at compile-time.

2. Step 2: Analyze the changeName method

   The @ts-ignore suppresses the TypeScript compile error for assigning to readonly. The code compiles to plain JavaScript, where objects are mutable, so this.name = newName; sets it to 'Bob' at runtime.

3. Final Answer:

   Bob -> Option D

4. Quick Check:

   Readonly is compile-time only; runtime allows changes [OK]

Hint: Readonly bypassed at runtime via @ts-ignore [OK]

Common Mistakes:

• Thinking readonly prevents runtime changes
• Expecting compilation error despite @ts-ignore
• Believing output remains 'Alice'

4. Identify the error in this TypeScript class:

class Product {
  readonly price: number;
  constructor() {
    // No assignment to price here
  }
}
const p = new Product();
p.price = 100;

medium

A. Error: readonly property 'price' is not initialized.

B. Error: Cannot assign to 'price' outside constructor.

C. No error, code runs fine.

D. Error: price must be declared as private.

Solution

1. Step 1: Check readonly property initialization

   The readonly property price must be assigned during declaration or in the constructor.

2. Step 2: Analyze constructor and assignment

   The constructor does not assign price, so TypeScript reports an error about missing initialization.

3. Final Answer:

   Error: readonly property 'price' is not initialized. -> Option A

4. Quick Check:

   Readonly props must be initialized at declaration or constructor [OK]

Hint: Always initialize readonly properties in constructor or declaration [OK]

Common Mistakes:

• Trying to assign readonly property after construction
• Ignoring missing initialization error
• Confusing readonly with private

5. Given this class, how can you safely allow updating the version property while keeping id readonly?

class Document {
  readonly id: string;
  version: number;
  constructor(id: string, version: number) {
    this.id = id;
    this.version = version;
  }
}

hard

A. Keep id readonly and update version normally via a method.

B. Make both id and version readonly and update via constructor only.

C. Remove readonly from id to allow all updates.

D. Make version readonly and update id instead.

Solution

1. Step 1: Understand readonly usage

   The id should remain readonly to keep it fixed after creation.

2. Step 2: Allow version updates

   The version property is not readonly, so it can be updated safely anytime, for example via a method.

3. Final Answer:

   Keep id readonly and update version normally via a method. -> Option A

4. Quick Check:

   Readonly for fixed, normal for changeable [OK]

Hint: Readonly for fixed props, normal for changeable ones [OK]

Common Mistakes:

• Making all properties readonly and losing flexibility
• Removing readonly from fixed properties
• Confusing which property should be updated