The event loop is a programming construct that waits for and dispatches events or messages in a program. It's fundamental to non-blocking and asynchronous programming.

How It Works

┌─────────────────────────────────────┐
│           Event Loop                │
│  ┌─────────────────────────────┐    │
│  │      Event Queue            │    │
│  │  [Event1] [Event2] [Event3] │    │
│  └─────────────────────────────┘    │
│              ↓                      │
│  ┌─────────────────────────────┐    │
│  │   Process One Event         │    │
│  │   (run to completion)       │    │
│  └─────────────────────────────┘    │
│              ↓                      │
│         Loop back                   │
└─────────────────────────────────────┘

Key characteristics:

• Runs continuously, processing events from a queue
• Single-threaded execution (one event at a time)
• Enables non-blocking operations without threads
• Common in JavaScript (browser & Node.js), Python asyncio, libuv

Example: JavaScript Event Loop

console.log('Start');
setTimeout(() => console.log('Timeout'), 10);
Promise.resolve().then(() => console.log('Promise'));
console.log('End');

// Output: Start, End, Promise, Timeout

Why this order?

1. Synchronous code runs first (Start, End)
2. Microtasks (Promises) run before macrotasks
3. Macrotasks (setTimeout) run last

Event Loop vs Thread Pool