React+ Typescript: Deep Dive into "readonly"

Type-Level Immutability in TypeScript with readonly

💡Intro

When you want to make sure no one mutates your types, readonly is your tool.

🧠Context

Unlike as const, which applies at the value level, readonly applies immutability at the type level. It prevents accidental mutations in objects, arrays, and class properties.

📌Intro

In React apps, readonly ensures props, API response types, and shared objects remain untouched.

❓Why Use This?

• Prevents runtime bugs by forbidding mutation.
• Useful for API response modeling.
• Guarantees immutability in large codebases.

📝With vs Without Example

// Without readonly interface User { name: string; age: number; } let user: User = { name: "Arka", age: 22 }; user.age = 23; // Allowed 

// With readonly interface User { readonly name: string; readonly age: number; } let user: User = { name: "Arka", age: 22 }; user.age = 23; // Error: cannot assign 

📌Real Life Use Cases

React Props

type ProfileProps = { readonly name: string; readonly skills: readonly string[]; }; function Profile({ name, skills }: ProfileProps) { // Cannot mutate skills inside return <div>{skills.join(", ")}</div>; } 

API Response

type ApiResponse = { readonly id: number; readonly status: "success" | "error"; }; 

readonly ensures compile-time safety. It does not make runtime data truly immutable, it just prevents mutation in code.