Just another very quick blog post, this time about loading JavaScript files in web pages, and just a few quick tips, and gotcha’s about it.
This is by no means a definitive list, but if you can do most of these things, then you’re certainly on the right track!

• Do not put ‘document.write’ into JS, the browser will have to to stop while it works out where to put this in the DOM, and how to render it. It will slow down your page.
  To make things worse, if you put the JS at the bottom of the page, and put document.write in, you make it even slow, by making it the last thing the browser does.
• Load JS files from a CDN or a different domain. However, the general limit is 4 domains in total (XHTML, CSS, JS, IMAGES etc). However, most people find that 2 is the best amount for the trade off between DNS lookups and extra concurrency.
• Make JavaScript external – allowing the browser to cache full requests, rather than including it in the XHTML, and increasing the page request size
• Related to the previous point, but make the external domains contain no cookies
• Minify the JS
• Put JS files at the bottom of the page
  or
• Use the DEFER attribute on SCRIPT tags (although remember this is not supported by Firefox)
• Use tools to help you find any performance problems, bottlenecks:
  Google PageSpeed – http://code.google.com/speed/page-speed/docs/using.html
  Yahoo YSlow – http://developer.yahoo.com/yslow/

Some links, and related articles:

• http://sixrevisions.com/web-development/10-ways-to-improve-your-web-page-performance/
• http://developer.yahoo.com/performance/rules.html
• http://oreilly.com/catalog/9780596802806
• http://www.die.net/musings/page_load_time/