Why Readonly properties in Typescript? - Purpose & Use Cases

The Big Idea

What if a tiny mistake could silently break your app? Readonly properties stop that from happening!

The Scenario

Imagine you have a list of important settings in your app that should never change once set. You write code to update them manually everywhere, hoping no one accidentally changes them later.

The Problem

This manual approach is risky and tiring. You might forget to protect some settings, causing bugs that are hard to find. It's like leaving your house keys under the doormat--anyone can change things without you knowing.

The Solution

Readonly properties let you mark data as unchangeable after creation. The TypeScript compiler then stops any accidental changes, giving you peace of mind and safer code without extra effort.

Before vs After

✗ Before

const config = { apiUrl: 'https://api.com' };
config.apiUrl = 'https://hack.com'; // Oops, changed!

✓ After

const config: { readonly apiUrl: string } = { apiUrl: 'https://api.com' };
config.apiUrl = 'https://hack.com'; // Error: cannot assign to 'apiUrl' because it is a read-only property

What It Enables

It enables you to write safer programs by preventing accidental changes to important data, making bugs easier to avoid.

Real Life Example

Think of a user profile where the user ID should never change after creation. Using readonly properties ensures this ID stays the same throughout the app.

Key Takeaways

Readonly properties protect important data from accidental changes.

They help catch errors early during coding, not later in production.

Using them makes your code more reliable and easier to maintain.

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