How do closures enable Data encapsulation/ Data Privacy

0-2 years experience

1. 1Write a function that returns an object with `increment` and `getValue` methods, where the internal counter cannot be accessed directly. How would you implement it using closures?
2. 2If you create a variable inside a function and return an inner function that uses it, can code outside the outer function modify that variable? Explain what happens.

2-5 years experience

1. 1You need to add a caching layer to an API client using closures to keep the cache private. Walk me through how you'd structure it and why closures help.
2. 2A teammate refactored a module that used a closure for private state, but now the state leaks and is being mutated from other modules. What could have gone wrong and how would you fix it?
3. 3Explain the trade‑offs between using a closure‑based module pattern versus ES6 classes for encapsulating data in a large codebase.

5-8 years experience

1. 1Our microservice uses a shared in‑memory store implemented with closures for per‑request data. Under high load we see memory growth. How would you diagnose and redesign to avoid leaks while keeping data private?
2. 2Design a plugin system where each plugin gets its own private configuration using closures. What considerations around performance and isolation would you address?
3. 3When migrating legacy code that relies on closure‑based privacy to a TypeScript codebase, what patterns would you adopt to preserve encapsulation without sacrificing type safety?

8+ years experience

1. 1Across several teams we have many utilities that use closures for private state. As we move to a monorepo, how would you establish guidelines or abstractions to ensure consistent encapsulation and avoid accidental state sharing?
2. 2We plan to expose a public SDK that must hide internal implementation details. How would you leverage closures at the library boundary versus other mechanisms, and what are the long‑term maintenance implications?
3. 3Consider a scenario where a closure captures a large object for privacy, but the object is also needed elsewhere. How would you architect the system to balance memory efficiency, data privacy, and testability at scale?