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JavaScript
is in the middle of a massive comeback and developers are abusing it.
JavaScript addons and widgets are becoming are the norm for large sites
and are so easy to install, in large numbers and quantities. For
example Mashable loads nearly a
mega byte in JavaScript files. In this article I will go through some
tips to help you optimize your JavaScript and reduce both the load on
your servers and make your pages respond faster.
Cut code
The first and biggest thing you should do in optimizing your
JavaScript is to cut down on the size, cut out functions that don't do
a lot and try and reduce the size of your file. This goes along side
with the question do you need a framework? Think it
through, is your site big enough to warrant loading a full framework
(typically in excess of 3000 lines), if you just want to do some very
simple functions write them in JavaScript without a framework.
If you really need to load a framework use the ones available on the Google AJAX libraries API,
most frameworks are available. These are just the frameworks hosted on
Google, they work of the principle that if one website uses these files
then the user will still have the files in the browser cache and won't
need load them again.
Minimize
Run your code through a JavaScript minimiser, this will shrink all
the files to be really small there are a number of popular ones
available (this is also applicable for CSS):
Expires headers
Set your expires headers, this should be standard for all your
static content like CSS and Images as well. A browser may need to make
many requests to your server in order to get all the content, by using
an expires header you make those components cacheable for a set period
of time, therefore the browser will need to make less HTTP requests.
For a more detailed explanation, as well other components which need
improving reading the Yahoo Developer Network Guide on optimisation.
Loading Scripts Asynchronously
Browsers are only single threaded, this means that it can only
really load one thing at a time. However there are ways around this,
and ways you can load multiple files without locking up the browser.
Give Steve Souders articles on Loading Scripts without Blocking
if you are interested in implementing this. I am currently working on a
prototype based class to do this which will be published on this blog
in the coming months.
Defer JavaScript Loading
Steve Souders talks about deferring JavaScript loading in his series about high performance websites.
It's quite a simple idea really, don't load up the JavaScript until you
require it. Therefore if your JavaScript is not needed on page load
this should be deferred to be loaded up later.
Evaluate
Make sure you evaluate your performance when trying to optimize JavaScript. I personally love the Firefox plugin FireBug, and it's plugin YSlow.
However there are lots of other methods and tools you can use to
evaluate you performance, the best by far is the firebug profiler. Micheal Sync has a fantastic tutorial on the FireBug profiler which is well worth a read.
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