Readonly properties in Typescript - Mini Project: Build & Apply

Readonly properties

📖 Scenario: You are creating a simple program to store information about a book. Some details about the book should not be changed once set, like its title and author.

🎯 Goal: Build a TypeScript program that uses readonly properties to protect the book's title and author from being changed after creation.

📋 What You'll Learn

Create an interface with readonly properties

Create an object that implements the interface

Try to modify a readonly property (should cause error if uncommented)

Print the book's title and author

💡 Why This Matters

🌍 Real World

Readonly properties are useful when you want to protect important data from accidental changes, like product IDs, user IDs, or configuration settings.

💼 Career

Understanding readonly properties helps you write safer and more predictable code, which is important in professional software development to avoid bugs.

1

Create an interface with readonly properties

Create an interface called Book with two readonly properties: title and author, both of type string.

Typescript

// Your code here

Hint

Use the readonly keyword before the property name inside the interface.

2

Create a book object implementing the interface

Create a constant called myBook of type Book and assign it an object with title set to "The TypeScript Guide" and author set to "Jane Doe".

Typescript

interface Book {
  readonly title: string;
  readonly author: string;
}

// Your code here

Hint

Use const myBook: Book = { title: "The TypeScript Guide", author: "Jane Doe" };

3

Try to modify a readonly property

Add a line that tries to change myBook.title to "New Title". Comment out this line to avoid errors but keep it in the code to show the attempt.

Typescript

interface Book {
  readonly title: string;
  readonly author: string;
}

const myBook: Book = {
  title: "The TypeScript Guide",
  author: "Jane Doe"
};

// Your code here

Hint

Write myBook.title = "New Title"; and comment it out with // at the start.

4

Print the book's title and author

Write a console.log statement to print the book's title and author in the format: "Title: The TypeScript Guide, Author: Jane Doe".

Typescript

interface Book {
  readonly title: string;
  readonly author: string;
}

const myBook: Book = {
  title: "The TypeScript Guide",
  author: "Jane Doe"
};

// myBook.title = "New Title"; // This line is commented out because it causes an error

// Your code here

Hint

Use a template string with console.log to print the message.

Practice

1. What does the readonly keyword do in TypeScript?

easy

A. Prevents a property from being changed after it is set

B. Allows a property to be changed anytime

C. Makes a property private and inaccessible

D. Deletes a property from an object

Solution

1. Step 1: Understand the purpose of readonly

   The readonly keyword is used to mark properties that cannot be changed after initialization.

2. Step 2: Compare options with the behavior

   Only Prevents a property from being changed after it is set correctly describes that the property cannot be changed after it is set.

3. Final Answer:

   Prevents a property from being changed after it is set -> Option A

4. Quick Check:

   readonly means no changes allowed after set [OK]

Hint: Readonly means no changes allowed after initial set [OK]

Common Mistakes:

• Thinking readonly allows changes anytime
• Confusing readonly with private
• Assuming readonly deletes properties

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

easy

A. const id: number;

B. readonly id: number;

C. private readonly id: number;

D. id readonly: number;

Solution

1. Step 1: Recall TypeScript syntax for readonly properties

   The correct syntax is to place readonly before the property name.

2. Step 2: Check each option

   readonly id: number; uses readonly id: number; which is correct. const id: number; uses const which is invalid inside class properties. private readonly id: number; adds private which is valid but not asked for. id readonly: number; has wrong order.

3. Final Answer:

   readonly id: number; -> Option B

4. Quick Check:

   Readonly keyword goes before property name [OK]

Hint: Put readonly before property name in class [OK]

Common Mistakes:

• Using const instead of readonly in class
• Placing readonly after property name
• Confusing access modifiers with readonly

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

class User {
  readonly name: string;
  constructor(name: string) {
    this.name = name;
  }
}
const user = new User('Alice');
user.name = 'Bob';
console.log(user.name);

medium

A. Alice

B. Bob

C. undefined

D. Compile-time error

Solution

1. Step 1: Understand readonly property behavior

   The property name is marked readonly, so it cannot be assigned a new value after construction.

2. Step 2: Analyze the assignment user.name = 'Bob'

   This line tries to change a readonly property, which causes a compile-time error in TypeScript.

3. Final Answer:

   Compile-time error -> Option D

4. Quick Check:

   Assigning to readonly property causes error [OK]

Hint: Changing readonly property causes compile error [OK]

Common Mistakes:

• Assuming readonly properties can be changed anytime
• Expecting runtime error instead of compile error
• Thinking output will be the new value

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

interface Point {
  readonly x: number;
  y: number;
}
const p: Point = { x: 10, y: 20 };
p.x = 15;
console.log(p.x);

medium

A. Property 'y' should be readonly too

B. Missing type annotation for 'p'

C. Cannot assign to 'x' because it is a readonly property

D. No error, code runs fine

Solution

1. Step 1: Check readonly property assignment

   The property x is readonly in the interface, so assigning p.x = 15 is not allowed.

2. Step 2: Identify the error message

   TypeScript will report an error: "Cannot assign to 'x' because it is a readonly property".

3. Final Answer:

   Cannot assign to 'x' because it is a readonly property -> Option C

4. Quick Check:

   Readonly properties cannot be assigned after initialization [OK]

Hint: Readonly properties cannot be reassigned after creation [OK]

Common Mistakes:

• Ignoring readonly and trying to assign anyway
• Thinking missing type annotation causes error
• Assuming no error occurs

5. You want to create a TypeScript class Config where the version property is set once during construction and never changes, but settings can be updated anytime. Which is the best way to declare these properties?

hard

A. readonly version: string; settings: object;

B. version: string; readonly settings: object;

C. readonly version: string; readonly settings: object;

D. version: string; settings: object;

Solution

1. Step 1: Understand property requirements

   The version should be readonly (set once, no changes). The settings should be mutable (can change anytime).

2. Step 2: Match options with requirements

   readonly version: string; settings: object; correctly marks version as readonly and leaves settings mutable. version: string; readonly settings: object; reverses roles. readonly version: string; readonly settings: object; makes both readonly. version: string; settings: object; makes both mutable.

3. Final Answer:

   readonly version: string; settings: object; -> Option A

4. Quick Check:

   Readonly for fixed, mutable for changeable properties [OK]

Hint: Readonly for fixed, normal for changeable properties [OK]

Common Mistakes:

• Making mutable properties readonly by mistake
• Not using readonly for fixed properties
• Confusing which property should be readonly