What makes Promise.then() run before setTimeout in JavaScript?

Microtask queue priority—Promises use it, setTimeout uses task queue. Microtasks drain first, every time.

How does await actually work under the hood?

It desugars to Promise.then(), splitting your async function and scheduling post-await code as a microtask. No blocking.

Sequential vs parallel await JavaScript—which is faster?

Parallel: Start all promises first (no await), then Promise.all(). Sequential chains wait times; parallel overlaps them.

🌐 Frontend & Web

Why Your JavaScript Async Code Isn't As 'Synchronous' As It Looks

Promises and await feel magical, but they're just queues fighting for stack time. Here's the gritty truth behind JavaScript async that no tutorial tells you straight.

theAIcatchup Apr 09, 2026 4 min read

JavaScript event loop diagram showing call stack, task queue, and microtask queue

⚡ Key Takeaways

Microtasks (Promises) always run before macrotasks (setTimeout)—that's why .then() wins. 𝕏
Await splits functions into microtasks; fire promises early for true parallelism. 𝕏
Event loop prioritizes: stack → all microtasks → one macrotask → repeat. 𝕏

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#async await #call-stack #event-loop #javascript #javascript-async #javascript-event-loop #microtasks #promises #promises-microtasks

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