Debounce / Async Request Coalescing

Problem Implement two related utilities. debounce(fn, delay) returns a function that postpones calling fn until delay ms have passed since the last invocation. Separately, build a request de-duplicator so that rapid identical fetches share a single in-flight request instead of firing many.

Input / Output

• Input: a function to debounce (plus a delay), and a request key + fetcher for the coalescer.
• Output: a debounced wrapper; and a fetch wrapper that returns the same promise for identical concurrent calls.

Constraints

• Support cancellation of a pending debounced call.
• Identical concurrent fetches must issue exactly one network request (single-flight).

Example

• Typing quickly into a search box issues one debounced search after typing stops.
• Two identical fetchUser(1) calls made before the first resolves share one promise and one request.