Sunday 30 August 2009

Compression Comparison

Comparing Compressor Tools

There are many Javascript compressor tools online and they all do wonderful jobs. What makes the tool I made slightly different is that it allows you to customise a number of compression options which can aid you in getting the best compression rate possible. Whilst most tools offer a simple crunch method and maybe a pack method (changing the code to run through an eval statement to obfuscate the code) they don't allow you to do some simple things that can make a lot of difference such as:

Renaming global objects that are used frequently to reduce size e.g window, document, navigator.

Renaming your own global objects that you may use frequently.

Renaming your own commonly accessed functions to short names.

Replacing calls to document.getElementById with a call to a single letter function e.g $ or G.

These 4 options used together could drastically alter the compression rate of your script.

Also if you have a small script then choosing to pack it as well as crunch or minify it will most likely increase the size of the output rather than compress it. Packing maybe worthwhile if you really want to hide your codes purpose from any user but its totally pointless as anyone can reverse engineer a packed script with a simple line of Javascript either within the Error Console in Firefox or by using one of the unpacker tools available online.

Different Outputs

Also I note that on a few compressors the output may give a misleading impression of success to the user. If a file has been reduced in size by 30% it has been compressed by 30%. Some tools however will show the new file size as a ratio of the old size which would be 70% which is fine. However having a label that just says "Compressed" next to the figure of 70% may lead some people to believe their file has been compressed by 70% when in fact its only been compressed by 30%.

For example take this silly example of a function:

function tested(var2){
var fuv = "hello "
+ "mr smith "
+ "how are you ";
var pid = 1000
var ola = 3343


var rob = function(){


var donkey = function(e){
if(fuv == pid){
return true;

Now run it through my compressor

Which outputs the following which has reduced the size by 40.65%

function tested(a){var b="hello mr smith how are you ";var c=1000,d=3343;if(a==b){var rob=function(){addEvent(c,"click",function(e){var donkey=function(e){if(b==c){return true}}})}}}

And now this other compressor tool.

Which outputs the following code and in a box labelled "Compression" it has the value 63.67%.

function tested(A){var B="hello "+"mr smith "+"how are you ";var C=1000var D=3343if(A==B){var E=function(){addEvent(C,"click",function(e){var F=function(e){if(B==C){return true;}}})}}}

Now this is actually the size of the new code in relation to the old and not how much the code has been reduced by which is 36.33%. This is not the only tool that does this and I am sure most people will be aware what the figures actually mean. However because my tool does the opposite and shows the percentage that the new code is reduced by it may lead people to believe one tool has a better compression rate than another when in fact it doesn't.

I am not claiming my tool is perfect as it will never be as it uses regular expressions instead of a Java Engine however most other tools do this as well and I have spent a lot of time handling issues such as missing terminators which other tools like the one above misses. Douglas Crockfords JSMin will just add a new line when a missing terminator is found whereas my tool adds the missing terminator. Other tools will just assume the code has been validated and checked before submission which of course is the best way to avoid any errors at all.

Compression with GZIP

Whats the benefit of minimising your script as compared to using GZIP?

Well GZIP may offer superior compression but its a way of delivering the content to the browser which then uncompress it. It uses server side processing to generate the compressed files which may or may not be a problem depending on the load and the type of file being served.

With minification the code can stay compressed on the server as well as the client plus it will run on older browsers. You also have the benefit that certain minification techniques should actually aid client side performance by reducing processing power e.g reducing the amount of variable declarations or reducing string concatenation by combining the strings together into one variable (see my compressed output)

There is nothing stopping you minifying as well as using GZIP. So if you haven't looked into compression then you should do as its a very simple and easy way of increasing your sites performance.

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