Elixir's Task.async_stream/3, living at its JS-idiomatic home: maps fn over the source with at most maxConcurrency invocations in flight, lazily — nothing starts until a consumer pulls, and the window refills only as results are taken. Results keep source order by default (ordered: false yields completion order — faster when element durations vary). Each element may deadline (timeoutMs) into a declared Failure('AsyncTimeout') element — the railway, not a crash; the element's signal fires so cancellation-aware work stops. A throw from fn stays a bug. Failure elements from the source pass through unmapped.

const doubled = await Stream.asyncStream([1, 2, 3, 4], (n) => n * 2, { maxConcurrency: 2 }).collect();
doubled; // [2, 4, 6, 8] — at most 2 in flight at any moment