Explain AWS access control and frontend typing

You are interviewing for a full-stack role. Answer the following conceptual questions. ## Part A — Cloud / AWS 1. **VPS vs EC2** - What is a VPS conceptually, and how does an AWS EC2 instance map to (or differ from) a traditional VPS? - What are common operational concerns (networking, storage, scaling, security) when running services on EC2? 2. **S3 pre-signed URLs** - What is an S3 pre-signed URL? - Describe a secure flow for **client-side upload** (browser/mobile directly to S3) using pre-signed URLs. - What should the backend validate or constrain (object key, content type, size, expiration, bucket policy)? 3. **Permissions / IAM basics** - Explain IAM **users**, **groups**, **roles**, and **policies**. - What is the difference between **identity-based** and **resource-based** policies? - Give an example of granting the minimum permissions needed to upload an object to a specific S3 prefix. ## Part B — React fundamentals 1. Explain what React **hooks** are and why they exist. 2. Walk through the purpose and typical use cases of common hooks, e.g.: - `useState`, `useEffect`, `useMemo`, `useCallback`, `useRef` 3. What are common pitfalls with `useEffect` dependencies and stale closures, and how do you avoid them? ## Part C — TypeScript fundamentals 1. Explain **structural typing** vs **nominal typing**. 2. Provide a small example showing how TypeScript’s type compatibility works under structural typing. 3. If you wanted nominal-like behavior in TypeScript, what are common patterns to achieve it?