2 snippets across 2 stacks — JavaScript, TypeScript
Also written as freeze object
Object.freeze(obj)
Object.seal(obj)
Object.isFrozen(obj)const API = Object.freeze({
BASE_URL: "https://api.example.com",
VERSION: 2,
});
API.VERSION = 3; // silently fails (throws in strict mode)
console.log(API.VERSION); // 2Note freeze() prevents all changes. seal() allows modifying existing properties but not adding/removing. Both are shallow -- nested objects remain mutable.
// readonly is compile-time only
const arr: readonly number[] = [1, 2, 3];
(arr as number[]).push(4); // No runtime protectioninterface Config {
readonly apiKey: string;
readonly maxRetries: number;
}
const config: Config = { apiKey: "secret", maxRetries: 3 };
// TypeScript prevents this:
// config.apiKey = "changed"; // Error
// But at runtime, nothing stops this:
(config as any).apiKey = "changed"; // Works at runtime!
console.log(config.apiKey); // "changed"
// For real immutability, use Object.freeze:
const safeConfig = Object.freeze({ apiKey: "secret", maxRetries: 3 });
// safeConfig.apiKey = "x"; // Runtime TypeError + compile error// readonly is erased at runtime — use Object.freeze for true immutabilityNote readonly, Readonly<T>, and ReadonlyArray are all compile-time only. They are stripped during compilation and provide zero runtime enforcement. Code that bypasses the type system (any casts, JavaScript interop) can still mutate. For sensitive data, combine readonly with Object.freeze or structuredClone.