I’m currently doing some optimization work on a large web project. I’m already doing JavaScript file combining, minification and compression. But I’m confused on one point.
For a number of non-technical reasons, my users are about 50% each IE7 and IE8. After doing some research, I’m getting the impression that IE7 loads the JavaScript files sequentially and IE8 loads them in parallel. I understand that going forward that this will not be an issue with more modern browsers (IE9+, FF, Chrome, etc).
Is this an accurate statement? If yes, then what is best practice for loading the files?
That statement is correct, but you should remember that even modern browser will make only a limited number of connections to the same server. So when your page, scripts, css and images are all on the same server, the browser may load only 2 or 4 of those at a time. Therefor it may be a good idea to add a subdomain or a different domain for scripts to trick the browser and make it load the scripts alongside with the images.
An even simpler solution is to merge all scripts into one script. You can do this ‘on the fly’ or cache it. You can even minimize the scripts (which means comments and whitespace are stripped and variable names are shortened). You shouldn’t minimize and combine the original scripts, but you can cache the combined/minimized scripts so they won’t need to be minimized with each request.
If you do this, you reduce traffic, and your browser will only need one request for the file, eliminating the overhead of multiple sequential requests.
See this MSDN blog article which shows some other tricks for script loading.