Monthly Archives: May 2009

As you begin developing more complex client-side functionality, managing the size and shape of your JavaScript includes becomes a key concern. It’s all too easy to accidentally end up with hundreds of kilobytes of JavaScript spread across many separate HTTP requests, significantly slowing down your initial page loads.

To combat this, it’s important to combine and compress your JavaScript. While there are useful standalone tools and HttpHandler based solutions to the problem already, none of them work quite how I prefer. Instead, I’m going to show you my dead-simple method for automatically compressing and combining script includes.

To accomplish that in this post, we will select a compression utility, learn how to use it at the command line, explore a useful automation feature in Visual Studio, and apply that to keep scripts combined and compressed with no ongoing effort.

Selecting a JavaScript compression tool

The first thing we’ll need is a utility to compress our JavaScript. There are many utilities available, ranging from YUI Compressor to Dean Edwards’ Packer, each with its own strengths and weaknesses.

YUI Compressor is powerful, but requires a Java runtime be available during the build process. Packer is popular for its Base62 encoding mode, however that form of compression carries a non-trivial performance tax on the client-side.

In terms of simplicity, it’s hard to beat Douglas Crockford’s JSMin. It requires no command line options, no runtimes or frameworks, and accepts input directly from standard input (which will be useful for us later).

One common concern about JSMin is that it outputs less compact code than YUI Compressor and Packer on their most aggressive settings. However, this is a bit of a red herring. When gzipped, the result of all three boil down to almost exactly the same size across the wire. Since you should always serve your JavaScript with gzip compression at the HTTP level, this initial “disadvantage” is moot.

Using JSMin from the command line

Using JSMin is very straightforward. For example, say we have the following, well-commented JavaScript and want to minify it:

// how many times shall we loop? 
var foo = 10;
 
// what message should we use? 
var bar = 'Encosia';
 
// annoy our user with O(foo) alerts! 
for (var i = 0; i < foo; i++) {
  alert(bar);
}

Assuming that JavaScript is in a file called AlertLoop.js, this command line usage of JSMin will minify it and output it to the console:

jsmin < AlertLoop.js

What this does is run jsmin and feed the contents of AlertLoop.js into standard input. It’s the same as if you had run jsmin and then typed all that JavaScript on the command line.

Similarly, this usage does the trick if you want to redirect that output to a file:

jsmin < AlertLoop.js > AlertLoop.min.js

The minified output is less than half the size of the original. Not bad!

Note: If you’re wondering about the upper ASCII characters preceding the minified script, they’re nothing to be concerned about. Because I had created AlertLoop.js in Visual Studio, it was saved as UTF-8 by default and those characters are the UTF BOM (thanks to Oleg, Sugendran, and Bart for clarification).

Set up project directories

Before we get to the next steps, we need to define a structure for our project. The one shown to the right works for simple projects.

Within the website project, the important takeaway is that the JavaScript files to be compressed are all in the same directory and named with a *.debug.js pattern.

Outside of the website, notice the “tools” directory which contains a copy of JSMin. I think we can all agree that executables should not be included within a website project if possible. That would just be begging for trouble.

However, I do suggest including an external tools directory and JSMin executable in your project’s source control. You never want to create a scenario where someone can’t perform a checkout and then a successful build immediately afterward.

Automation: Visual Studio earns its keep

To automate script compression as part of the build process, I suggest using a build event. There are perfectly legitimate alternatives, but I prefer having a tangible file sitting on disk and having that compression process automated. So, “building” the minified JavaScript include(s) as part of the build process makes the most sense to me.

Build events may sound complicated, but they aren’t at all. Build events are simply a mechanism for executing command line code before and/or after your project is compiled.

For our purposes, a post-build event is perfect. Additionally, we can specify that it should only run the build event if the project builds successfully. That way we avoid wasting unnecessary time on minifying the JavaScript when there are build errors.

Setting up a build event in Visual Studio

To add build events, right-click on your project and choose properties. In the properties page that opens, click on the “Build Events” tab to the left. You’ll be presented with something similar to this:

Note: If you’re using Visual Basic, there will be no build events tab in the project properties. Instead, look for a build events button on the “Build” tab, which allows access the same functionality.

You can type commands directly in the post-build field if you want, but clicking the “Edit Post-build” button provides a better editing interface:

The interface’s macro list is especially useful. In particular, the ProjectDir macro will be handy for what we’re doing. $(ProjectDir) placed anywhere in a build event will be replaced with the actual project path, including a trailing backslash.

For example, we can use it to execute JSMin.exe in the hierarchy described above:

$(ProjectDir)..toolsjsmin.exe

Or, reference that same project’s js directory:

$(ProjectDir)js

Putting it all together: Minify a single file

Now that we’ve covered how to use JSMin at the command line and how to execute command line scripts as part of Visual Studio builds, putting it all together is easy.

For example, to minify default.debug.js, this post-build event will do the trick:

"$(ProjectDir)..toolsjsmin" <
"$(ProjectDir)jsdefault.debug.js" >
"$(ProjectDir)jsdefault.min.js"

(The line breaks are for readability here. The command in your actual build event must not contain them, or it will be interpreted as separate commands and fail.)

The quotes are important, in case $(ProjectDir) happens to include directories with spaces in their names. Since you never know where this project may eventually be built at, it’s best to always use the quotes.

*Really* putting it together: Combine files

I did promise more than just compression in the post’s title. Combining scripts is just as important as compression, if not more so. Since JSMin takes its input from stdin, it’s easy to roll scripts together for minification into a single result:

type "$(ProjectDir)js*.debug.js" |
"$(ProjectDir)..toolsjsmin" >
"$(ProjectDir)jsscript-bundle.min.js"

This build event would combine all of our *.debug.js scripts, minify the combined script bundle, and then output it in a new file named script-bundle.min.js.

This is great if you want to combine your most commonly used jQuery plugins into a single payload, for example. A reduction in HTTP requests usually provides a nice improvement in performance. This is especially true when you’re dealing with JavaScript, because the browser blocks while script references load.

Dealing with dependencies

Cross-dependencies between scripts is one issue that requires extra consideration when combining. Just the same as ordering script includes incorrectly, bundling scripts together in the wrong order may cause them to fail.

One relatively easy way to handle this is to give your scripts prefixes to force the correct order. For example, the source sample below includes this set of JavaScript files:

jQuery-1.3.2.debug.js
jQuery-jtemplates.debug.js
default.debug.js

Combining these and referencing the result will fail, because default.debug.js is sorted ahead both jQuery and the plugin by default. Since default.debug.js depends on both of those, this is a big problem. To fix this, rename the files with prefixes:

01-jQuery.debug.js
05-jQuery-jtemplates.debug.js
10-default.debug.js

Now it will work perfectly.

Any system of alphanumeric prefixes will work, but if you use a numeric system, be sure to pad the numbers with leading zeroes. Otherwise, the default sort ordering may catch you off guard (e.g. 2-file.js sorts ahead of 11-file.js through 19-file.js).

To debug, or not to debug

Now that we have the minification process under control, one final issue to address is how to keep this from complicating our development workflow.

While editing these scripts, we certainly don’t want to be forced to recompile every time we make a change to the JavaScript. After all, one of the nice things about JavaScript is that it doesn’t require precompilation. Even worse, using a JavaScript debugger against minified files is a nightmare I wouldn’t recommend to anyone.

The easiest way I know of to ensure that the correct scripts are emitted for both scenarios is to check the IsDebuggingEnabled property of the HttpContext:

<head>
<% if (HttpContext.Current.IsDebuggingEnabled) { %>
  <script type="text/javascript" src="js/01-jquery-1.3.2.debug.js"></script>
  <script type="text/javascript" src="js/05-jquery-jtemplates.debug.js"></script>
  <script type="text/javascript" src="js/10-default.debug.js"></script>
<% } else { %>
  <script type="text/javascript" src="js/js-bundle.min.js"></script>
<% } %>
</head>

When the web.config’s compilation mode is set to debug, the *.debug.js versions of the files are referenced, and the auto-minified bundle otherwise. Now we have the best of both worlds.

Conclusion

I hope you’ll find that this technique is a good compromise between the tedium of using manual minification tools and the overwrought complexity of setting up some of the more “enterprisey” automation solutions.

One not-so-obvious benefit that I’ve noticed stems from minification’s automatic comment stripping. Without worry about your comments burdening the size of the client-side payload or being distributed across the Internet, you’re more likely to comment your JavaScript well. Dealing with a dynamic language, sans-compiler, I find that comments are often crucial to maintainability.

This is one of those problems with quite a few perfectly legitimate solutions. What do you think of this solution? How do you normally handle this?

Get the source

For demonstration, I took my jQuery client-side repeater example and applied this technique. Having several JavaScript includes (one that’s full of comments), it’s a perfect candidate for combining and compression.

One particular thing to notice in this example is the use of numeric prefixes to order the JavaScript includes, as mentioned earlier. This naming scheme is crucial when dealing with interdependent scripts. If the scripts are combined in the wrong order, your functionality will break just the same as if you had used script reference tags in the wrong order.

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Originally posted at Encosia. If you’re reading this elsewhere, come on over and see the original.

Automatically minify and combine JavaScript in Visual Studio