Loading JavaScript efficiently is key to fast and smooth web pages. Two powerful attributes, async and defer, help you control how scripts load and execute, avoiding delays and improving user experience. But what’s the difference, and when should you use each?
What is async?
- Scripts marked with async download in parallel with HTML parsing.
- As soon as the script is ready, it executes immediately, possibly interrupting HTML parsing.
- Script execution order is not guaranteed with async.
Example:
Use async for scripts like ads or analytics that run independently and don’t rely on the DOM or other scripts.
What is defer?
- Scripts marked with defer also download in parallel but wait to execute until the entire HTML is parsed.
- Scripts run in the order they appear in the HTML.
- Execution happens just before the DOMContentLoaded event.
Example:
Use defer when scripts depend on the DOM or on each other, ensuring reliable execution order without blocking render.
Why Are They Important?
By default, scripts block page rendering, making the page load slower and frustrating users. Both async and defer fix this by allowing the browser to continue parsing content while scripts download.
Choosing the right attribute helps:
- Speed up page load times
- Improve perceived performance
- Prevent errors due to scripts running too early
Developer Tips
- Place async or defer scripts in the <head> to start downloading early.
- Avoid using both async and defer together on the same script.
- Test your page behavior after using these attributes to catch timing issues.
When to Use async
- For independent scripts that don’t depend on other scripts or DOM, such as analytics or ads.
- When you want to speed up page load by running scripts as soon as they are ready, without blocking HTML parsing.
- When execution order among scripts is not important.
When to Use defer
- For scripts that interact with the DOM or depend on other scripts.
- When scripts need to execute in the order they appear in the HTML document.
- When you want scripts to run only after the full HTML document is parsed, ensuring the DOM is ready.
Difference between async and defer
| Feature | Async | Defer |
|---|---|---|
| Script Download | Downloads in parallel with HTML parsing | Downloads in parallel with HTML parsing |
| Script Execution Timing | Executes as soon as it finishes downloading (may interrupt parsing) | Executes after HTML parsing is complete |
| Execution Order | Not guaranteed (executes whenever ready) | Guaranteed to preserve order as in the document |
| Best Use Cases | For independent scripts like ads, analytics, or widgets | For scripts that rely on DOM or need ordered execution |
| DOM Availability During Execution | DOM may still be loading | DOM is fully parsed and ready |
Conclusion
Understanding the difference between async and defer empowers developers to build faster, smoother web applications with improved user experience. Using the right attribute improves page load speed and avoids script execution errors or race conditions.