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I was reading a tutorial and the author mentioned to include JavaScript files near the closing body tag (</body>) in HTML.
For what type of functionality should I not declare/define JavaScript include in the head section? It makes sense to me include JavaScript like Google Analytics near the closing body tag. Where should I be careful in defining JavaScript include near the closing body tag?
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JavaScript in <head> or <body> You can place any number of scripts in an HTML document. Scripts can be placed in the <body> , or in the <head> section of an HTML page, or in both.
You can place the <script> tags, containing your JavaScript, anywhere within your web page, but it is normally recommended that you should keep it within the <head> tags.
The best practice is to put JavaScript <script> tags just before the closing </body> tag rather than in the <head> section of your HTML. The reason for this is that HTML loads from top to bottom. The head loads first, then the body, and then everything inside the 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.
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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.
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