This content originally appeared on DEV Community and was authored by Arka Chakraborty
Type-Level Immutability in TypeScript with readonly
đź’ˇIntro
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.
đź“ŚIntro
In React apps, readonly ensures props, API response types, and shared objects remain untouched.
âť“Why Use This?
- Prevents runtime bugs by forbidding mutation.
- Useful for API response modeling.
- Guarantees immutability in large codebases.
đź“ťWith 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
đź“ŚReal 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.
This content originally appeared on DEV Community and was authored by Arka Chakraborty
Arka Chakraborty | Sciencx (2025-09-15T08:01:00+00:00) React+ Typescript: Deep Dive into “readonly”. Retrieved from https://www.scien.cx/2025/09/15/react-typescript-deep-dive-into-readonly/
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