The event loop runs your JavaScript on a single thread, so anything synchronous that takes a long time blocks it — and while blocked, Node serves no other request, timer, or callback. Blocking is almost always CPU-bound work or a *Sync API, never properly-awaited async I/O.
*Sync) APIs — fs.readFileSync, fs.writeFileSync, child_process.execSync, zlib.gzipSync. They wait inline instead of handing off to libuv.map/sort/filter, matrix math, image/video processing, parsing/serializing huge structures.JSON.parse / JSON.stringify — synchronous and O(n); a multi-MB payload freezes the loop for the whole parse.crypto.pbkdf2Sync, bcrypt sync mode, hashing large buffers on the main thread.while (true), unbounded recursion, for over millions of items with work per item.// All of these block the loop — no other request is served meanwhile:
const data = fs.readFileSync("big.json"); // sync I/O
const obj = JSON.parse(hugeString); // large parse
const hash = crypto.pbkdf2Sync(pw, salt, 1e6, 64, "sha512"); // sync crypto
for (let i = 0; i < 1e9; i++) { /* CPU */ } // tight loop
Wrapping CPU work in an
asyncfunction does not help —asynconly yields atawait; the synchronous body still runs to completion on the loop thread.
perf_hooks.monitorEventLoopDelay() or libraries like blocked-at; rising lag means something is hogging the thread.node --prof, clinic doctor/clinic flame, Chrome DevTools CPU profile to find the hot synchronous frame.*Sync call (fs.promises.readFile, crypto.pbkdf2, zlib.gzip).worker_threads pool, a child process, or an external queue/service.setImmediate/await so the loop can breathe between batches.const { Worker } = require("node:worker_threads");
// Offload the expensive hash so the loop stays responsive.
const worker = new Worker("./hash-worker.js", { workerData: { pw, salt } });
In Node, one blocking call punishes every concurrent client, not just the request that caused it — latency spikes, health checks time out, and the process looks "hung" under load. Interviewers ask this to see whether you can name the concrete blockers (*Sync, big JSON.parse, sync crypto, ReDoS, CPU loops), detect them with event-loop-lag tooling, and fix them by going async or offloading to worker threads. It's the single most common Node performance failure in production.
A library of IT interview questions with detailed answers — from Junior to Senior.
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