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
Medium
75 questions, 25–45 minutes each
Implement, then defend your edge cases, performance and tests.
Promises & async
Promise combinators and async helpers implemented from scratch.
- 36Implement Promise.all() from scratchFree
- 37Implement
Promise.allSettled() - 38Implement
Promise.race() - 39Implement
Promise.any()withAggregateError - 40Implement
Promise.resolve()andPromise.reject() - 41Implement
promisify()for callback-based APIs - 42Implement
asyncMap()preserving result order - 43Implement
asyncFilter()with asynchronous predicates - 44Implement
asyncReduce()with sequential execution - 45Implement a
retry()utility supporting maximum attempts and delays