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 async concurrency
Schedulers, cancellation, backpressure and resilience patterns.
- 101Implement a Promise library with
.then(),.catch(), chaining, and asynchronous handler scheduling - 102Implement a task scheduler with concurrency limits, priorities, cancellation, and starvation prevention
- 103Implement a
mapLimit()withAbortSignal, ordered results, and configurable error-handling policies - 104Implement
asyncMap()over an async iterable with bounded concurrency and backpressure - 105Implement an asynchronous dependency graph executor where tasks run only after their dependencies complete
- 106Implement a circuit breaker with
CLOSED,OPEN, andHALF_OPENstates - 107Implement a distributed-style API client rate limiter coordinating multiple browser tabs
- 108Implement a single-flight token refresh mechanism: 10 failed requests trigger only one refresh, followed by safe retriesFree
- 109Implement a streaming upload manager supporting chunking, bounded concurrency, progress, retry, and cancellation
- 110Implement a resilient polling engine with exponential backoff, visibility awareness, timeout, and cancellation