Defer vs Async in JavaScript — The Script Tag Mistake Everyone Makes
Dhruv Burada
Your page shows a blank white screen on reload?
Or your app suddenly stops working after you shuffle around script files?
Most of the time, the culprit is how you’re loading your JavaScript files.
The Usual Mistake
Many developers just drop scripts like this:
<head>
<script src="app.js"></script>
</head>
What happens?
The browser stops parsing HTML until
app.jsis fully downloaded and executed.Result: your page takes longer to show up (a.k.a. render-blocking).
This is why you often see a white screen or a sluggish first load.
The Fix: defer
Enter the defer attribute:
<head>
<script src="app.js" defer></script>
</head>
What happens now:
HTML keeps parsing while the script is being fetched.
The script only runs after the DOM is fully parsed.
No more white screens caused by blocking JavaScript.
That’s why defer is your best friend for loading essential scripts that depend on the DOM.
But What About async?
Another option is async:
<head>
<script src="analytics.js" async></script>
</head>
Key difference:
asyncalso downloads in parallel, but executes as soon as it’s ready, even if the HTML is still parsing.This is great for scripts that don’t depend on the DOM or other scripts (like analytics, ads, or trackers).
Quick Comparison
| Attribute | When Script Runs | Best For |
| (none) | Blocks HTML, runs immediately | Old-school, not recommended |
defer | After HTML parsing is complete | Main app code, DOM-dependent scripts |
async | As soon as file is ready | Independent scripts (analytics, ads) |
TL;DR
Avoid using plain
<script src="..."></script>in<head>.Use
deferfor your main JavaScript.Use
asyncfor independent scripts.
This simple change leads to faster page loads and fewer white screen issues.
If you found this helpful, follow along for more beginner-friendly insights on JavaScript, the MERN stack, and real-world development practices.
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