6 JavaScript Event Loop Patterns That Eliminate Async Bugs in Production

Async bugs are not random. They are event loop ordering issues. This post shows 6 patterns you can apply immediately to stop guessing and start controlling execution. 1. Replace setTimeout(fn, 0) with queueMicrotask for deterministic ordering You think you are deferring work to “next tick.” You are actually pushing it behind every microtask. Before setTimeout(() => { updateUI(); }, 0); After queueMicrotask(() => { updateUI(); }); Microtasks always run before timers. This removes 10 to 50ms of unpredictable delay when promise chains are long. 2. Move side effects out of stale closures after setState Reading state right after setting it gives you old values. This is not React. This is closure timing. Before function handleClick() { setCount(count + 1); sendMetric(count); // stale } After function handleClick() { const next = count + 1; setCount(next); sendMetric(next); } You eliminate an entire class of “sometimes wrong” bugs with one local variable. 3. Collapse multiple async stat