Should Jquery code go in header or footer?
Put Scripts at the Bottom
The problem caused by scripts is that they block parallel downloads. The HTTP/1.1 specification suggests that browsers download no more than two components in parallel per hostname. If you serve your images from multiple hostnames, you can get more than two downloads to occur in parallel. While a script is downloading, however, the browser won't start any other downloads, even on different hostnames. In some situations it's not easy to move scripts to the bottom. If, for example, the script uses document.write to insert part of the page's content, it can't be moved lower in the page. There might also be scoping issues. In many cases, there are ways to workaround these situations.
An alternative suggestion that often comes up is to use deferred scripts. The DEFER attribute indicates that the script does not contain document.write, and is a clue to browsers that they can continue rendering. Unfortunately, Firefox doesn't support the DEFER attribute. In Internet Explorer, the script may be deferred, but not as much as desired. If a script can be deferred, it can also be moved to the bottom of the page. That will make your web pages load faster.
EDIT: Firefox does support the DEFER attribute since version 3.6.
Sources:
- http://www.w3schools.com/tags/att_script_defer.asp or better:
- http://caniuse.com/#feat=script-defer
All scripts should be loaded last
In just about every case, it's best to place all your script references at the end of the page, just before </body>.
If you are unable to do so due to templating issues and whatnot, decorate your script tags with the defer attribute so that the browser knows to download your scripts after the HTML has been downloaded:
<script src="my.js" type="text/javascript" defer="defer"></script>
Edge cases
There are some edge cases, however, where you may experience page flickering or other artifacts during page load which can usually be solved by simply placing your jQuery script references in the <head> tag without the defer attribute. These cases include jQuery UI and other addons such as jCarousel or Treeview which modify the DOM as part of their functionality.
Further caveats
There are some libraries that must be loaded before the DOM or CSS, such as polyfills. Modernizr is one such library that must be placed in the head tag.
Only load jQuery itself in the head, via CDN of course.
Why? In some scenarios you might include a partial template (e.g. ajax login form snippet) with embedded jQuery dependent code; if jQuery is loaded at page bottom, you get a "$ is not defined" error, nice.
There are ways to workaround this of course (such as not embedding any JS and appending to a load-at-bottom js bundle), but why lose the freedom of lazily loaded js, of being able to place jQuery dependent code anywhere you please? Javascript engine doesn't care where the code lives in the DOM so long as dependencies (like jQuery being loaded) are satisfied.
For your common/shared js files, yes, place them before </body>, but for the exceptions, where it really just makes sense application maintenance-wise to stick a jQuery dependent snippet or file reference right there at that point in the html, do so.
There is no performance hit loading jquery in the head; what browser on the planet does not already have jQuery CDN file in cache?
Much ado about nothing, stick jQuery in the head and let your js freedom reign.