node-asynclisted

# Node.js Async Patterns Node.js runs on a single thread. Everything async works because the event loop defers I/O — but that model breaks the moment you block the thread or mismanage promises. Most Node.js production issues trace back to one of a handful of async mistakes. ## Never block the event loop The event loop handles every request. If you block it — with a CPU-intensive loop, a synchronous file read, or heavy JSON parsing — every in-flight request stalls until you're done. ```ts // Bad: blocks the event loop for every caller app.get("/report", (req, res) => { const result = fs.readFileSync("large-file.csv"); // synchronous — blocks res.send(process(result)); }); // Good: yields to the event loop app.get("/report", async (req, res) => { const result = await fs.promises.readFile("large-file.csv"); res.send(process(result)); }); ``` **CPU-bound work** (parsing, hashing, compression) belongs in a Worker Thread, not the event loop. Offload it: ```ts import { Worker } from "worker_threads"; function runInWorker(data: unknown): Promise<unknown> { return new Promise((resolve, reject) => { const worker = new Worker("./worker.js", { workerData: data }); worker.on("message", resolve); worker.on("error", reject); }); } ``` ## Handle every rejected promise In modern Node.js (v15+), an unhandled promise rejection crashes the process. In older versions it silently swallows the error — often worse. ```ts // Bad: rejection is swallowed if sendEmail