Glossary term
What is event loop? Meaning for developers
In JavaScript Core
A tiny loop that watches the call stack and, whenever it is empty, moves the next waiting callback from a queue onto the stack.
Learn it properly · JavaScript CoreLesson 14: Async JS and the Event LoopCall stack, Web APIs, callback queue, microtask queue and the event loop that ties them together, stepped tick by tick — plus async/await, where rendering fits, starvation, and why setTimeout(0) is not 0.In System Design
One thread handling many tasks by never waiting idly. Example: Node.js and NGINX serving thousands of connections.
Learn it properly · System DesignLesson 4: Processes, Threads & Concurrency for Backend EngineersMost backend work is waiting. How one server juggles thousands of requests — processes, threads, event loops and lightweight threads — and the race conditions, deadlocks and pool limits that appear the moment it does.In Backend in Depth
Node's single thread that runs JavaScript callbacks one at a time. If you block it, every request waits.
Learn it properly · Backend in DepthLesson 10: Node.js Performance Basics: Event Loop Blocking, Streams, WorkersWhy one heavy CPU job can stop every request, plus streams for large data and worker threads.In Full-Stack Questions
The part of Node.js (and browsers) that picks up finished work and runs its callbacks one at a time.
Learn it properly · Full-Stack QuestionsLesson 6: Node.js & Express Interview QuestionsEvent loop, streams, workers, Express 5, security, graceful shutdown and performance.In Node.js From Zero
The loop in Node that waits for finished work and runs one callback at a time, each to the end.
Learn it properly · Node.js From ZeroLesson 1: The Event Loop: Timers, Promises and the Poll PhaseThe loop phases, microtasks versus macrotasks, process.nextTick, setImmediate, and how a blocked loop freezes everything. Traced and run.