What is the JavaScript event loop?

It's the scheduler juggling macrotasks (like timeouts), microtasks (Promises), and browser steps like rendering—ensuring single-threaded JS stays responsive.

Why does setTimeout(0) not run immediately?

Because it queues as a macrotask, only after current task and all microtasks finish—no interrupting running JS.

How does requestAnimationFrame fit in?

It runs before render, after microtasks, syncing code to display refresh for smooth visuals.

🌐 Frontend & Web

JavaScript Event Loop: Experiments That Shatter Async Myths

Why does setTimeout with zero delay ghost you? This series of experiments reveals JavaScript's event loop secrets, turning async confusion into crystal-clear reasoning.

theAIcatchup Apr 10, 2026 4 min read

Visual diagram of JavaScript event loop phases: macrotasks, microtasks, rendering

⚡ Key Takeaways

JavaScript is single-threaded: Tasks run to completion, async defers via queues. 𝕏
Microtasks (Promises) always precede macrotasks (setTimeout), explaining priority. 𝕏
Rendering waits for loop phases—use rAF to sync UI updates without jank. 𝕏

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#Microtasks Macrotasks #async javascript #javascript-event-loop #microtasks #requestAnimationFrame

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