Where should I declare JavaScript files used in my page? In <head></head> or near </body>?
It will often be argued that for speed purposes you should put script tags right at the end of the document (before the closing body tag). While this will result in the fastest page load, it has some serious downsides.
Firstly, a common idiom with Webpage development is to have a header file, a footer file and your content in the middle. To keep unnecessary JavaScript code to a minimum, you'll often want to put code snippets in individual pages.
If you include jQuery, for example, at the end of the document, your jQuery code snippets (like document ready stuff) must happen after that. That can be awkward from a development point of view.
Secondly, in my experience, because the page load is faster, you can end up noticing certain effects being applied because the page has already loaded by the time they are applied.
For example, if you put a table in a document and right before the body close tag put:
$(function() {
$("tr:nth-child(odd)").addClass("odd");
});
with appropriate styling, that effect being applied will often be visible. Personally I think that makes for a bad user experience potentially. I think often you're better off having the page load slightly slower (by putting scripts at the top) if you don't get disconcerting visual effects.
I generally advocate effective caching strategies so you only have to download JavaScript files when they change, as in Supercharging JavaScript in PHP (but the principles apply to any language, not just PHP) and still putting scripts at the top. It's far more convenient.
By putting them in the <head/> you force the browser to download the files before it can render a page. That causes the perceived load time to slow down.
By placing them in the footer, right before the closing body tag, the browser will not load them until it reaches that point in the parsing of the HTML. That means that the scripts will run later in the page load process but will not block the asset download and rendering process.
Which works best is up to you and how you develop your code.
The Yahoo YSlow tool has advice on this:
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.
Google pagespeed have some nice explanation on how to parallelize downloading of scripts.
Still their advice is to put them in the head of your page.