How does the Event Loop work in Node.js? | Node.js Interview Questions

What You'll Learn

How the event loop works
Event loop phases
Understanding async behavior

Event Loop Overview

The Event Loop is what allows Node.js to perform non-blocking I/O operations despite JavaScript being single-threaded.

Event Loop Phases

code.jsJavaScript

┌───────────────────────────┐
┌─>│           timers          │ (setTimeout, setInterval)
│  └─────────────┬─────────────┘
│  ┌─────────────┴─────────────┐
│  │     pending callbacks     │ (I/O callbacks)
│  └─────────────┬─────────────┘
│  ┌─────────────┴─────────────┐
│  │       idle, prepare       │ (internal use)
│  └─────────────┬─────────────┘
│  ┌─────────────┴─────────────┐
│  │           poll            │ (retrieve new I/O events)
│  └─────────────┬─────────────┘
│  ┌─────────────┴─────────────┐
│  │           check           │ (setImmediate)
│  └─────────────┬─────────────┘
│  ┌─────────────┴─────────────┐
└──┤      close callbacks      │ (socket.on('close'))
   └───────────────────────────┘

Example

code.jsJavaScript

console.log('Start');

setTimeout(() => console.log('Timeout'), 0);

setImmediate(() => console.log('Immediate'));

process.nextTick(() => console.log('NextTick'));

console.log('End');

// Output: Start, End, NextTick, Timeout/Immediate

Key Points

process.nextTick() runs before event loop continues
setImmediate() runs in check phase
setTimeout(0) runs in timers phase