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Async Patterns, Errors and EventEmitter

Node layers its own async conventions on top of promises: error-first callbacks, synchronous EventEmitters, process lifecycle events, AbortSignal-aware APIs and AsyncLocalStorage. Know how each behaves when things go wrong, because that is what crashes production services.

Key points

1
Error-first callbacks must be called exactly once and always asynchronously. Mixing sync and async callbacks ("Zalgo") makes ordering unpredictable; defer cache hits with process.nextTick.
2
util.promisify resolves with the first result only, forwards its own this (so bind methods), and returns fn[util.promisify.custom] when present, which is why promisify(setTimeout) is the timers/promises version.
3
emit() runs listeners synchronously in registration order on a snapshot of the listener list. A throwing listener stops the rest, and error with no listener throws. Async listeners' rejections are ignored unless captureRejections is on.
4
events.once rejects if error fires first, and events.on yields forever until aborted. Neither keeps the process alive, and neither does a timeout signal: only ref'd handles (sockets, timers, I/O) do.
5
Since Node 15 an unhandled rejection crashes the process (--unhandled-rejections=throw) unless an unhandledRejection listener exists. strict crashes even with one, and warn-with-error-code continues but exits with code 1.
6
After uncaughtException, log, clean up synchronously, exit non-zero and let a supervisor restart you. exit listeners cannot do async work; beforeExit fires only when the loop drains naturally, not on process.exit().
7
AsyncLocalStorage context follows the async resource that runs a callback. Libraries that queue callbacks (pools, batchers) must wrap them with AsyncResource.bind, or requests see each other's store.

Common traps

Removing a listener during emit() does not stop it running in that same emit; the change applies to the next one.
A plain setTimeout backoff ignores cancellation. Use timers/promises setTimeout(ms, undefined, { signal }) so an abort ends the wait immediately.
An entry ES module whose top-level await can never settle exits with code 13, often because an event was emitted before anyone awaited it.

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