Event Loop — Complete Guide
Event Loop — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of MEAN Stack Tutorial on Toolliyo Academy.
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MEAN Stack Tutorial · Lesson 32 of 100
Event Loop
Stack → Projects
Stack · 1 — Pieces · ~6 min · MEAN — Node.js & Express
What is this?
The event loop lets Node handle many connections by dequeuing callbacks when I/O completes — timers, promises, and HTTP handlers share one thread.
Why should you care?
MeanVerse devs must know why CPU-heavy JSON parsing blocks all API users on one worker.
See it live — copy this example
Paste into your MeanVerse project (Angular + Express + MongoDB), then run with ng serve / node / mongosh as noted.
const express = require('express');
const app = express();
app.get('/fast', (_req, res) => res.json({ ok: true }));
app.get('/slow-cpu', (_req, res) => {
const start = Date.now();
while (Date.now() - start < 3000) { /* blocks event loop */ }
res.json({ done: true });
});
app.listen(3000);What happened?
- While /slow-cpu spins CPU, /fast requests wait — same thread.
- Offload heavy work to worker threads or queue for banking batch jobs.
Practice next
- Hit /fast and /slow-cpu concurrently with curl.
- Observe /fast latency spike during CPU loop.
- Replace loop with setImmediate chunked processing.
- Add logging order: sync, Promise.resolve().then, setTimeout.
- Move bcrypt rounds to worker pool for auth.
Remember
Event loop schedules callbacks after I/O. CPU work blocks all concurrent requests. Use workers/queues for heavy MeanVerse jobs.
Statement PDF generation
One sync PDF lib froze payment API during month-end.
Outcome: Job moved to Bull queue — event loop stays free.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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