What is the purpose of the " readonly " modifier in TypeScript?

The 'readonly' modifier in TypeScript is primarily intended to ensure that properties of a class or interface cannot be modified after initialization. This helps maintain data immutability during programming, making the program more secure and predictable.

Using Scenarios Example:

Suppose we are developing a system with a User class that needs to include basic information such as the user's ID and username. These details must not be altered after creating the user object, as the ID and username are critical for user authentication and database operations. In this case, we can use the readonly modifier to ensure these properties remain unmodified after creation.

typescript
class User {
    readonly id: number;
    readonly username: string;

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

let user = new User(1, "ChatGPT");
console.log(user.id); // Output: 1
console.log(user.username); // Output: ChatGPT

// Attempt to modify property
user.id = 2; // TypeScript compilation error: Cannot assign to 'id' because it is a read-only property.

Summary:

In summary, the readonly modifier is a valuable feature provided by TypeScript. It enables developers to write more stable and secure code by enforcing that data remains immutable once set. This characteristic is particularly useful for scenarios requiring data immutability, such as authentication, configuration settings, or any other data that should not be modified after initialization.