IntermediateTECHNICAL
Walk me through how you would design and implement a feature that lets users upload images, store them, and then display optimized thumbnails on a dashboard. Cover front-end handling (validation, previews, error states), back-end API design, storage choice, and how you’d handle performance and security concerns (e.g., large files, malicious uploads).
Full Stack Developer
General

Sample Answer

I’d build a React front end with an `<input type="file" multiple>` wrapped in a drag-and-drop zone. On select, I validate type (MIME/extension whitelist: jpg/png/webp), size (e.g., max 5 MB), and count, then show client-side previews via `URL.createObjectURL`. Errors (wrong type/too large/network) are surfaced inline per file. Uploads use a signed-URL approach: the frontend requests a POST `/api/uploads/sign` (Node.js/Express or NestJS), then uploads directly to S3/GCS, avoiding backend payload bloat. The backend stores metadata (URL, ownerId, size, hash) in Postgres and triggers a thumbnail Lambda/Cloud Function using Sharp/Imagemagick to generate multiple sizes and store them under a CDN (CloudFront) path. I cap file size at the API gateway, scan uploads with a malware scanner (e.g., ClamAV or cloud service), enforce authZ (per-user access), and use caching + pagination on the dashboard to keep thumbnail loads under ~200 ms P95.

Keywords

Client-side validation, previews, and granular error states in ReactSigned URLs for direct-to-cloud uploads and a metadata APIAsync thumbnail generation using serverless functions and SharpSecurity controls: size limits, MIME checks, malware scanning, authZ
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