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?
There are two possibilities for truly unobtrusive scripts:
</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.
It's never so cut and dry - Yahoo recommends putting the scripts just before the closing </body> tag, which will create the illusion that the page loads faster on an empty cache (since the scripts won't block downloading the rest of the document). However, if you have some code you want to run on page load, it will only start executing after the entire page has loaded. If you put the scripts in the <head> tag, they would start executing before - so on a primed cache the page would actually appear to load faster.
Also, the privilege of putting scripts at the bottom of the page is not always available. If you need to include inline scripts in your views that depend on a library or some other JavaScript code being loaded before, you must load those dependencies in the <head> tag.
All in all Yahoo's recommendations are interesting but not always applicable and should be considered on a case-by-case basis.
As others have said, place it before the closing body html tags.
The other day we had numerous calls from clients complaining their sites were extremely slow. We visited them locally and found they took 20-30 seconds to load a single page. Thinking it was the servers performing badly, we logged on - but both web and sql servers were ~0% activity.
After a few minutes, we realised an external site was down, which we were linking to for Javascript tracking tags. This meant browsers were hitting the script tag in the head section of the site and waiting to download the script file.
So, for 3rd party/external scripts at least, I'd recommend putting them as the last thing on the page. Then if they were unavailable, the browser would at least load the page up until that point - and the user would be oblivious to it.
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