53Implement request deduplication for identical in-flight API calls

Three components mount at the same time and each calls getUser(42). Without care, that is three identical network requests.

Implement dedupe(fn, getKey). It returns a function with the same signature as the async fn. While a call for a given key is in flight, further calls with the same key get the same promise instead of calling fn again. Once that promise settles, the entry is removed, so the next call makes a fresh request.

getKey(...args) defaults to JSON.stringify(args).

const getUser = dedupe((id) => fetch(`/api/users/${id}`).then((r) => r.json()));
const [a, b, c] = await Promise.all([getUser(42), getUser(42), getUser(42)]); // one request

Requirements

• Concurrent calls with the same key share one call to fn and one promise.
• Different keys run independently.
• After the shared promise settles (fulfilled or rejected), the next call calls fn again. This is deduplication, not caching.
• A failure is delivered to every waiting caller.
• If fn throws synchronously, the call rejects and nothing stays stuck in flight.