JavaScript - head, body or jQuery?
It is possible to download javascripts in parallel by doing something like this:
(function () {
var ele = document.createElement('script');
ele.src = "https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js";
ele.id = "JQuery";
ele.onload = function () {
//code to be executed when the document has been loaded
};
document.getElementsByTagName('head')[0].appendChild(ele);
})();
In the example it downloads minified JQuery v1.7.2 from Google, this is a good way to download JQuery since downloading it from Google is like using a CDN and if the user has been on a page that used the same file it might be cached and therefor doesn't need to be downloaded
There is a really good Google tech talk about this here http://www.youtube.com/watch?v=52gL93S3usU&feature=plcp
Although common practice, putting script tags in the head is not usually a good idea, as it holds up the rendering of your page until those scripts have been downloaded and processed (barring your use of async or defer and the browser supporting them).
The usual recommendation is to put script tags at the very end of the body tag, e.g., just before </body>. That way, all of the DOM elements above the script will be accessible (see links below). One caveat on that is that there can be a brief moment when your page has been at least partially-rendered but your scripts not processed (yet), and if the user starts interacting with the page, they may do something to raise an event that your script hasn't had time to hook yet. So you need to be aware of that. This is one reason for progressive enhancement, which is the idea that the page will work without JavaScript, but work better with it. If you're doing a page/app that just won't work without JavaScript at all, you might include some inline script at the top of the body tag (e.g., <script>minimal code here</script>) that hooks any bubbling events on document.body and either queues them for action when your script is loaded, or just asks the user to wait.
Using features like jQuery's ready is fine, but not really necessary outside of libraries (e.g., if you're in control of where the script tags will be, you don't usually need to use it; but if you're writing a jQuery plug-in that needs to do something on load [which is relatively rare, normally they just wait to be called], you usually do).
More reading:
- YUI Best Practices for Speeding Up your Website
- Google on when the DOM will be ready
Most recommended method is to put it before </body> tag. Yahoo performance article also suggests that other than YSlow and Page Speed addons by Yahoo and Google respectively.
Quoting from Yahoo article linked above:
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.
When you put scripts in <head> tag, the browsers goes for them thereby keeping other stuff on hold until scripts are loaded which users will perceive like slow loading of the page. This is why you should put scripts at the bottom.
As for:
$(document).ready(function(){/*Code goes here*/});
It is fired when DOM is available and ready to be manipulated. If you put your code at the end, you won't necessarily need this but usually this is needed because you want to do something as soon as DOM is available for use.