Why Readonly class properties in Typescript? - Purpose & Use Cases

The Big Idea

What if you could stop bugs before they even happen by locking important data in your code?

The Scenario

Imagine you create a class to represent a user profile. You want some details, like the user ID, to never change after the profile is created. But without a way to lock these details, anyone can accidentally change them later in the code.

The Problem

Manually checking every place in your code to avoid changing important properties is slow and error-prone. It's easy to forget and accidentally overwrite values, causing bugs that are hard to find.

The Solution

Readonly class properties let you mark certain values as unchangeable after they are set once. This means the TypeScript compiler will stop you if you try to change them, keeping your data safe and your code more reliable.

Before vs After

✗ Before

class User {
  id: number;
  constructor(id: number) {
    this.id = id;
  }
}

const user = new User(1);
user.id = 2; // Oops, changed by mistake!

✓ After

class User {
  readonly id: number;
  constructor(id: number) {
    this.id = id;
  }
}

const user = new User(1);
user.id = 2; // Error: Cannot assign to 'id' because it is a read-only property.

What It Enables

This lets you build safer programs where important data stays constant, preventing bugs and making your code easier to trust and maintain.

Real Life Example

Think of a banking app where an account number should never change once created. Using readonly properties ensures the account number stays the same throughout the app.

Key Takeaways

Readonly properties prevent accidental changes to important data.

They help catch errors early during coding, not at runtime.

Using them makes your code safer and easier to maintain.

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