Developing Your Site For Performance : JavaScript/File-Related Optimization

    November 17, 2004

This is the third part of a five part article. Read part 2 here.

More and more sites rely on JavaScript to provide navigational menus, form validation, and a variety of other useful things. Not surprisingly, much of this code is quite bulky and begs for optimization. Many of the techniques for JavaScript optimization are similar to those used for markup and CSS. However, JavaScript optimization must be performed far more carefully because, if it is done improperly, the result is not just a visual distortion, but potentially a broken page! We start with the most obvious and easiest improvements and then move on to ones that require greater care.

11. Remove JavaScript comments

Except for the <!– //–> masking comment, all JavaScript comments indicated by // or /* */ can safely be removed, as they offer no value to end users (except for the ones who want to understand how your script works).

12. Remove white space in JavaScript

Interestingly, white space removal in JavaScript is not nearly as beneficial as it might seem. On the one hand, code like

x = x + 1;

can obviously be reduced to


However, because of the common sloppy coding practice of JavaScript developers failing to terminate lines with semi-colons, white space reduction can cause problems. For example, given the legal JavaScript below which uses implied semi-colons


a simple white space remover might produce


which would obviously throw an error. If you add in the needed semi-colons to produce


you actually gain nothing in byte count. We still encourage this transformation, however, since Web developers who provided feedback on the Beta version of the w3compiler found the “visually compressed” script more satisfying (perhaps as visual confirmation that they are looking at transformed rather than original code). The also liked the side benefit of delivering more obfuscated code.

13. Perform code optimizations

Simple ideas like removing implied semi-colons, var statements in certain cases, or empty return statements can help to further reduce some script code. Shorthand can also be employed in a number of situations, for example


can become


However, be careful, as it is quite easy to break your code unless your optimizations are very conservative.

14. Rename user-defined variables and function names

For good readability, any script should use variables like sumTotal instead of s. However, for download speed, the lengthy variable sumTotal is a liability and it provides no user value, so s is a much better choice. Here again, writing your source code in a readable fashion and then using a tool to prepare it for delivery shows its value, since remapping all user defined variable and function names to short one- and two-letter identifiers can produce significant savings.

15. Remap built-in objects

The bulkiness of JavaScript code, beyond long user variable names, comes from the use of built-in objects like Window, Document, Navigator and so on. For example, given code like


you could rewrite it as


This type of remapping is quite valuable when objects are used repeatedly, which they generally are. Note however, that if the window or navigator object were used only once, these substitutions would actually make the code bigger, so be careful if you are optimizing by hand. Fortunately, many JavaScript code optimizers will take this into account automatically.

This tip brings up a related issue regarding the performance of scripts with remapped objects: in addition to the benefit of size reduction, such remappings actually slightly improve script execution times because the objects are copied higher up into JavaScript’s scope chain. This technique has been used for years by developers who write JavaScript games, and while it does improve both download and execution performance, it does so at the expense of local browser memory usage.

File-Related Optimization

The last set of optimization techniques is related to file and site organization. Some of the optimizations mentioned here might require server modifications or site restructuring.

16. Rename non-user accessed dependent files and directories

Sites will often have file names such as SubHeaderAbout.gif or rollover.js for dependent objects that are never accessed by a user via the URL. Very often, these are kept in a standard directory like /images, so you may see markup like

<img src="/images/SubHeaderAbout.gif">

or worse

<img src="../../../images/SubHeaderAbout.gif">

Given that these files will never be accessed directly, this readability provides no value to the user, only the developer. For delivery’s sake it would make more sense to use markup like

<img src="/0/a.gif">

While manual file-and-directory remapping can be an intensive process, some content management systems can deploy content to target names, including shortened values. Furthermore, the w3compiler has a feature that automatically copies and sets up these dependencies. If used properly, this can result in very noticeable savings in the (X)HTML files that reference these objects and can also make reworking of stolen site markup much more difficult.

17. Shorten all page URLs using a URL rewriter

Notice that the previous step does not suggest renaming the host files like products.html, which would change markup like


to something like


The main reason is that end users will see a URL like, rather than the infinitely more usable

However, it is possible to reap the benefits of file name reduction in your source code without sacrificing meaningful page URLs if you combine the renaming technique with a change to your Web server’s configuration. For example, you could substitute p.html for products.html in your source code, but then set up a URL rewriting rule to be used by a server filter like mod_rewrite to expand the URL back into a user friendly value. Note that this trick will only put the new URL in the user’s address bar if the rewrite rule employs an “external” redirect, thereby forcing the browser to re-request the page. In this case, the files themselves are not renamed as the short identifiers are only used in the source code URLs.

Because of the reliance on URL rewriting and the lack of widespread developer access to, and understanding of, such server-side tools as mod_rewrite, even an advanced tool like the w3compiler does not currently promote this technique. However, considering that sites like Yahoo! actively employ this technique for significant savings, it should not be ignored, as it does produce noticeable (X)HTML reduction when extremely descriptive directory and file names are used in a site.

18. Remove or reduce file extensions

Interestingly, there really is little value to including file extensions such as .gif, .jpg, .js, and so on. The browser does not rely on these values to render a page; rather it uses the MIME type header in the response. Knowing this, we might take

<img src="images/SubHeaderAbout.gif">

and shorten it to

<img src="images/SubHeaderAbout">

Or, if combined with file renaming, you might have

<img src="/0/sA">

Don’t be scared away by how strange this technique looks at first, as your actual file will still be sA.gif. It is just the end user who won’t see it that way!

In order to take advantage of this more advanced technique, however, you do need to make modifications to your server. The main thing you will have to do is to enable something called “content negotiation,” which may be native to your server or require an extension such as mod_negotation for Apache or Port80’s PageXchanger for IIS. The downside to this is that it may cause a slight performance hit on your server. However, the benefits of adding content negotiation far outweigh the costs. Clean URLs improve both security and portability of your sites, and even allow for adaptive content delivery whereby you can send different image types or languages to users based upon their browser’s capabilities or system preferences! See “Towards Next Generation URLs” by the same authors for more information.

Note: Extension-less URLs will not hurt your search engine ranking. Port80 Software, as well as major sites like the W3C, use this technique and have suffered no ill effects.

19. Restructure <script> and <style> inclusions for optimal number of requests

You will often see in the <head> of an HTML document markup like

<script src="/scripts/rollovers.js"></script>
<script src="/scripts/validation.js"></script>
<script src="/scripts/tracking.js"></script>

In most cases, this should have been reduced to

<script src="/0/g.js"></script>

where g.js contains all the globally used functions. While the break-up of the script files into three pieces makes sense for maintainability, for delivery it does not. The single script download is far more efficient than three separate requests, and it even reduces the amount of needed markup. Interestingly, this approach mimics the concept of linking in a traditional programming language compiler.

20. Consider cacheability at the code level

One of the most important improvements to site performance that can be made is to improve cacheability. Web developers may be very familiar with using the tag to set cache control, but (apart from the fact that meta has no effect on proxy caches) the true value of cacheability is in found in its application to dependent objects such as images and scripts. To prepare your site for improved caching, you should consider segmenting your dependent objects according to frequency of change, storing your more cacheable items in a directory like /cache or /images/cache. Once you start organizing your site this way, it will be very easy to add cache control rules that will make your site clearly “pop” for users who are frequent visitors.

So you now have 20 useful code optimization tips to make your site faster. One by one, they may not seem very powerful, but taken together you will see an obvious improvement in site delivery. Next, we will focus primarily on caching, explaining how it is generally misused and how you can significantly improve performance with just a few simple changes.

*Originally published at

Thomas Powell is founder of PINT, Inc. and a lecturer in the Computer Science
department at University of California San Diego. His articles have appeared in
serveral magazines and sites, including Network World, Internet Week
and ZDNet. He has also published numerous books on Web technology and design,
including the best-selling Web Design: The Complete Reference. Visit

Joe Lima is the Director of Product Development for Port80 Software. He has
worked for a variety of Internet, wireless and software development companies,
specializing in research and development for server-centric technologies. Visit