Browser vs. Node.js Event Loop: Key Differences Every JS Developer Should Know

JavaScript is single-threaded but handles high concurrency through the Event Loop, which delegates heavy tasks like I/O and timers to system threads while queuing callbacks for the main thread. Both browsers and Node.js use this mechanism, but their implementations differ significantly in structure and optimization. The browser Event Loop includes a rendering phase targeting ~60Hz frame updates, where microtask bloat can freeze the UI, and requestAnimationFrame is preferred for visual updates. Node.js splits its macrotask queue into distinct phases — Timers, Poll, and Check — and adds process.nextTick(), which executes before standard Promise microtasks and can starve the Event Loop if called recursively. Understanding these differences is critical for writing performant, non-blocking JavaScript in both environments.