Rabin R
Interview Prepintermediate

Explaining the event loop in an interview

Microtasks vs macrotasks, and why a Promise callback always beats a setTimeout(0).

Rabin R

Jun 28, 2026 · 5 min read

Introduction

The call stack runs synchronous code. When it empties, the loop drains the microtask queue (Promises, queueMicrotask, MutationObserver) completely, then takes exactly one macrotask (timers, I/O, events) and repeats.

That is why a resolved Promise callback runs before a setTimeout(0) queued at the same time.

  • Microtasks: Promise.then, await continuations, queueMicrotask.
  • Macrotasks: setTimeout, setInterval, DOM events, message channel.
  • Rendering happens between macrotasks, after microtasks drain.