There are two common ways that people use to reduce the download size of a JavaScript file: compression and minification.

When you host your website on a Windows Server using IIS 7.0, you can configure IIS to automatically compress your JavaScript files using GZIP compression – which can provide a significant improvement on performance and the download size of files. However, you can get additional performance benefits by both compressing and minifying your JavaScript files. Steve Sounders describes these additional benefits in his excellent book High Performance Web Sites.

In addition to releasing Microsoft Ajax Library (Preview 6), we are today also releasing a new (free) Microsoft Ajax Minifier utility that can help reduce the size of your JavaScript files considerably.  It was created by Ron Logon who works on the MSN team. You can download the Microsoft Ajax Minifier from the ASP.NET CodePlex website for free. 

The following screenshot demonstrates the results of minifying the standard jQuery library using various minification tools such as Douglas Crockford’s JSMin, Dean Edward’s Packer, and the YUI Compressor. The bottom two files were minified using the Microsoft Ajax Minifier utility. Notice that the Microsoft Ajax Minifier has reduced jQuery from 125 KB to only 53 KB.

Figure 10

The Microsoft Ajax Minifier supports two levels of minification: normal and hypercrunched. When you use normal minification, the Microsoft Ajax Minifier removes all unnecessary whitespace, comments, curly braces, and semi-colons.  When you enable hypercrunching, the Microsoft Ajax Minifier becomes more aggressive in reducing the size of a JavaScript file, and it minifies local variable names and removes unreachable code.

Here’s a sample of a JavaScript file:

Figure 11

Here’s what the JavaScript file looks like after it has been minified with the Microsoft Ajax Minifier (with hypercrunching enabled):

Figure 12

Notice that all unnecessary whitespace has been removed. Notice also that the function parameters firstValue and secondValue have been renamed to b and a.

The Microsoft Ajax Minifier download includes the following components:

ajaxmin.exe – A command-line tool for minifying JavaScript files.

ajaxmintask.dll – A MSBuild task for minifying JavaScript files in a Visual Studio project.

ajaxmin.dll – A component that you can use in your C# or VB.NET applications to minify JavaScript files.

After you install the Microsoft Ajax Minifier, you can use the Microsoft Ajax Minifier command-line tool to minify a JavaScript file from a command-prompt. 

You also have the option of adding the Microsoft Ajax Minifier as a custom MSBuild task to Visual Studio. Adding the Microsoft Ajax Minifier MSBuild task to your Visual Studio project file allows you to automatically minify all of the JavaScript files in your project whenever you perform a build, and enables you to perform minification in an automated way.