Using readonly in TypeScript: Ensuring Immutability and Data Integrity

Learn how the readonly keyword in TypeScript enforces immutability, ensuring that property values cannot be modified after assignment. Explore its use in class properties, interfaces, and type aliases to maintain data integrity and enhance code reliability.

TypeScript Readonly

The readonly keyword in TypeScript guarantees that a property's value cannot be modified after it's assigned. This is crucial for maintaining data integrity and preventing accidental modifications. By using readonly, you create immutable properties, enhancing code reliability and predictability.

When to Use readonly

• Class properties: Prevent accidental changes to object properties.
• Interface properties: Define contracts where properties should not be mutable.
• Type aliases: Create read-only versions of existing types.

Examples

class Person {
  readonly firstName: string;
  readonly lastName: string;

  constructor(firstName: string, lastName: string) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
}

No direct output for this example.

interface User {
  readonly id: number;
  email: string;
}

No direct output for this example.

type ImmutablePoint = Readonly<{ x: number; y: number }>;

No direct output for this example.

Deep Readonly

While readonly prevents direct modification of properties, it doesn't prevent modifications to nested objects. To create deeply immutable objects, use the DeepReadonly utility type:

import { DeepReadonly } from 'typescript';

interface Address {
  street: string;
  city: string;
}

interface User {
  name: string;
  address: Address;
}

const user: DeepReadonly = {
  name: 'John Doe',
  address: { street: '123 Main St', city: 'Anytown' }
};

// user.name = 'Jane Doe'; // Error
// user.address.street = '456 Elm St'; // Error

No direct output for this example.

Additional Considerations

• Initialization: readonly properties must be initialized either at declaration or within the constructor.
• Compile-Time Enforcement: TypeScript's type system enforces readonly restrictions at compile time.
• Runtime Behavior: While TypeScript prevents compile-time errors, it's essential to remember that readonly is a compile-time construct. JavaScript itself doesn't have a readonly concept.