Welcome back! I am Mihir, and in this lesson we will learn readonly class properties in TypeScript.

readonly means a property can be assigned during initialization, but cannot be reassigned later.

Basic readonly Property

class User {
  readonly id: number;
  name: string;

  constructor(id: number, name: string) {
    this.id = id;
    this.name = name;
  }
}

Valid:

const user = new User(1, "Mihir");
user.name = "Mihir Soni";

Invalid:

user.id = 2;

readonly with Default Value

class AppConfig {
  readonly appName: string = "CodeWithMihir";
}

Once the instance is created, appName cannot be reassigned.

readonly Constructor Parameter Property

You can use readonly directly in constructor parameters.

class Product {
  constructor(
    public readonly id: number,
    public name: string
  ) {}
}

This creates an id property and makes it readonly.

readonly Still Allows Reading

const product = new Product(101, "Keyboard");

console.log(product.id);

Readonly properties can always be read.

They just cannot be reassigned.

readonly Is Not Deep Immutability

class Cart {
  readonly items: string[] = [];
}

const cart = new Cart();
cart.items.push("Keyboard");

This is allowed because the items property still points to the same array.

Invalid:

cart.items = [];

readonly prevents reassigning the property, not mutating the object inside it.

When to Use readonly

Use readonly for values that should not change after creation.

Good examples:

• database IDs
• creation dates
• configuration names
• fixed labels
• injected dependencies

Quick Recap

• readonly prevents property reassignment.
• Readonly properties can be assigned during initialization or in the constructor.
• Constructor parameter properties can be readonly.
• readonly is not deep immutability.
• Use it for values that should stay stable after object creation.

Next up, we will learn Getters and Setters →.