deepFreeze<T> — Runtime Readonly

Object.freeze only freezes the top level of an object. Nested objects remain mutable:

const config = Object.freeze({ db: { host: 'localhost' } });
config.db = {};            // ❌ frozen — throws in strict mode
config.db.host = 'other';  // ✅ nested object is NOT frozen — mutates silently!

deepFreeze fixes this by recursively freezing all nested objects.

TypeScript angle

The compile-time equivalent is the DeepReadonly<T> utility type:

type DeepReadonly<T> = {
  readonly [K in keyof T]: T[K] extends object ? DeepReadonly<T[K]> : T[K];
};

// deepFreeze is the runtime enforcement of DeepReadonly:
function deepFreeze<T extends object>(obj: T): Readonly<T>

Note: TypeScript's built-in Readonly<T> is shallow (only marks top-level properties as readonly). DeepReadonly<T> is a custom recursive mapped type.

Implement

deepFreeze<T extends object>(obj: T): Readonly<T>

• Freeze obj with Object.freeze
• Recursively freeze all object-valued own properties
• Skip null values and already-frozen objects
• Return the same object (mutated in place, just frozen)