The Must-Solve 150.
Every question is a real implementation, debugging or architecture task. Work through the levels in order, or jump straight to what your next interview needs.
10 of 150 questions
Hard
50 questions, 45–90 minutes each
Senior-level correctness, architecture and production constraints.
Advanced JS, browser & architecture
Sandboxes, parsers, reactivity, offline sync and secure messaging.
- 121Implement a sandboxed JavaScript execution environment using an isolated iframe or worker, with a clearly defined security boundary
- 122Implement a simplified HTML parser and DOM tree builder
- 123Implement a reactive dependency system with automatic dependency trackingFree
- 124Implement a memory-efficient event bus with lifecycle cleanup and leak detection
- 125Implement an offline-first data synchronization queue using IndexedDB
- 126Implement an HTTP cache with ETag validation, stale-while-revalidate, and request deduplication
- 127Build a resumable upload client handling network interruption and server reconciliation
- 128Design a micro-frontend shell with independent deployment and shared dependency versioning
- 129Implement a secure postMessage communication protocol between a parent page and embedded iframe
- 130Build a browser-side analytics delivery system with batching, sampling, offline queueing, and deduplication