Unobtrusive JavaScript: <script> at the top or the bottom of the HTML code?
I've recently read the Yahoo manifesto Best Practices for Speeding Up Your Web Site. They recommend to put the JavaScript inclusion at the bottom of the HTML code when we can.
But where exactly and when?
Should we put it before closing </html> or after ? And above all, when should we still put it in the <head> section?
Answer
There are two possibilities for truly unobtrusive scripts:
- including an external script file via a script tag in the head section
- including an external script file via a script tag at the bottom of the body (before
</body></html>)
The second one can be faster as the original Yahoo research showed some browsers try to load script files when they hit the script tag and therefore don't load the rest of the page until they have finished. However, if your script has a 'ready' portion which must execute as soon as the DOM is ready you may need to have it in the head. Another issue is layout - if your script is going to change the page layout you want it loaded as early as possible so your page does not spend a long time redrawing itself in front of your users.
If the external script site is on another domain (like external widgets) it may be worth putting it at the bottom to avoid it delaying loading of the page.
And for any performance issues do your own benchmarks - what may be true at one time when a study is done might change with your own local setup or changes in browsers.