Asynchronous and Event-Driven Programming Questions

The programming model of a single asynchronous runtime: callbacks, promises and futures, async/await, event loops and task/microtask scheduling (including microtask starvation and where rendering fits), reactive streams and back-pressure, and how ordering, cancellation, timeouts, retries and error propagation behave across JavaScript, Node.js, mobile coroutine or async runtimes, and a comparison with Python asyncio and Java futures. Includes the async mechanics of a data fetch such as parallel versus sequential requests, partial failure, stale or out-of-order responses, abort, debounced live search, single-flight token refresh, and bounded concurrency; wrapping callback APIs as promises, streaming work through async consumers, moving heavy work off the main thread and back, and diagnosing unhandled rejections, race conditions and leaked subscriptions. Excludes message-queue and pub/sub system design, server throughput and thread-pool tuning, per-language threading and memory models, rebuilding standard APIs from scratch, and fetch caching or state architecture.