What is the difference in how the event loop works on the server and in the browser?

In the browser, the main engine of the Event Loop is the JavaScript engine (e.g., V8). Task processing occurs in the main, single thread. Tasks are divided into macrotasks (DOM events, timers, I/O) and microtasks (Promise, MutationObserver). The Event Loop cycle first executes all available microtasks after each macrotask.

On the server (Node.js), the Event Loop is implemented by the libuv library. It is multithreaded and uses a thread pool to perform blocking input/output operations (file operations, network requests to other hosts). The Event Loop in Node.js also has phases (timers, pending callbacks, poll, check, close callbacks), which are processed in a specific order. Microtasks are processed after each phase.

Key differences:

• Concurrency: Browser - single-threaded main thread. Node.js - single-threaded Event Loop + thread pool for I/O.
• Task sources: In the browser - DOM events, timers, network requests from browser API. In Node.js - file operations, network requests, timers.
• Phases: In Node.js, the Event Loop has explicitly defined phases; in the browser, the model is more abstract and focuses on macro/microtasks.

Aspect	Browser	Node.js
Event Loop Engine	Part of the JavaScript engine	libuv
I/O Processing	Browser APIs (Web APIs) via main thread	Thread pool from libuv
Event Loop Phases	Macro- and microtasks	Timers, Poll, Check, Close, etc.
I/O Sources	DOM, Network, Files (rarely)	Files, Network