TypeScript — Readonly Properties: Protect Data Integrity with Type Safety

Introduction – What Are Readonly Properties in TypeScript?

In TypeScript, the readonly modifier is used to make a property immutable after initialization. This means once a value is assigned to a readonly property—either during declaration or within a constructor—it cannot be reassigned later. This feature enhances data safety, promotes immutable design, and helps prevent accidental mutations in your codebase.

In this guide, you’ll learn:

• How to declare readonly properties in TypeScript
• Use cases in classes, objects, and arrays
• The difference between readonly and const
• Best practices and common mistakes

Why Use readonly?

• Prevent accidental changes to important data
• Enforce immutability for object properties
• Improve code clarity and intent
• Align with functional programming principles

Declaring Readonly Properties in Classes

Use the readonly keyword before property names in class definitions.

Example:

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

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

const item = new Product(101, "Laptop");
item.name = "Tablet";     //  Allowed
// item.id = 202;         //  Error: Cannot assign to 'id' because it is a read-only property

Explanation:

• id is immutable once set in the constructor.
• name remains mutable.

Readonly in Object Types

You can also use readonly in interfaces and type aliases.

interface User {
  readonly username: string;
  age: number;
}

const user: User = {
  username: "coder123",
  age: 25
};

user.age = 26;              //  Allowed
// user.username = "admin"; //  Error

Readonly Arrays

TypeScript supports readonly for arrays to prevent mutation methods like push, pop, splice, etc.

Example:

const colors: readonly string[] = ["red", "green", "blue"];
// colors.push("yellow"); //  Error

You can also use ReadonlyArray<T> syntax:

const numbers: ReadonlyArray<number> = [1, 2, 3];

readonly vs const – Key Differences

Real-World Use Cases

• API response models where IDs must remain constant
• Securing configurations that shouldn’t be altered
• Data models with immutable primary keys
• Maintaining array-based state without mutations

Common Mistakes & How to Avoid Them

Best Practices for Readonly Properties

• Use readonly to model immutable fields like id, createdAt, etc.
• Use ReadonlyArray<T> for protecting list data from mutations
• Combine with const to create fully immutable patterns
• Document intent clearly for future developers
• Don’t assume deep immutability—readonly is shallow

Summary – Recap & Next Steps

readonly in TypeScript is a simple yet powerful feature that promotes safe coding practices and defensive programming. By marking fields as immutable, you reduce bugs and make your codebase more predictable and secure.

Key Takeaways:

• readonly prevents reassignment of properties after initialization
• Use in classes, object types, and arrays
• Prefer for identifiers, config fields, and immutable structures
• Combine with const and immutability patterns for safer apps

Real-world relevance: Readonly properties are essential in data modeling, API contracts, state management, and class-based systems.

FAQs – Readonly in TypeScript

Can I reassign a readonly property after construction?
No. You can only assign a value once—during declaration or inside the constructor.

Is readonly a runtime feature?
No. It’s enforced at compile time only and removed from the JavaScript output.

Can I use readonly on methods?
No. readonly applies to properties, not functions. Use it to protect values, not behaviors.

Are readonly arrays deeply immutable?
No. Only the top-level array methods are restricted. Nested objects can still be modified unless typed as readonly too.

Can I combine readonly with other types?
Yes. You can use readonly with interface, type, and arrays for more controlled type definitions.