ARTICLE

JavaScript and CSS Minifier

Posted by Matt Perdeck Articles | ASP.NET Programming January 31, 2011
The CombineAndMinify package discussed here automatically speeds up the loading of JavaScript files, CSS files and or images (loaded from image tags or from CSS files). The result can be a dramatic improvement in ASP.NET performance.
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Introduction

The CombineAndMinify package discussed here automatically speeds up the loading of JavaScript files, CSS files and or images (loaded from image tags or from CSS files). The result can be a dramatic improvement in ASP.NET performance.

Installation is easy - just add a dll, update your web.config and add a small file. If you use IIS 6, you'll also need to update the configuration of IIS (no such update needed for IIS 7). Step by step installation instructions are in the installation section.
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Requirements

  • ASP.NET 3.5 or higher
  • IIS 6 or higher for your live site

Features

The features below can all be switched on and off individually via the web.config file (full description). If you just install the package and not do any further configuration, it only minifies and combines JavaScript and CSS files, and than only when the site is in Release mode.

Although the feature set below is extensive, there are many more ways to improve ASP.NET performance. My recently released book ASP.NET Site Performance Secrets shows how to pinpoint the biggest performance bottlenecks in your web site, using various tools and performance counters built into Windows, IIS and SQL Server. It then shows how to fix those bottlenecks. It covers all environments used by a web site - the web server, the database server, and the browser. The book is extremely hands on - the aim is to improve web site performance today, without wading through a lot of theory first.

CombineAndMinify features:

  • Minifies JavaScript and CSS files. Minification involves stripping superfluous white space and comments. Only JavaScript and CSS files that are loaded from the head sections of your pages are minified.
     
  • Correctly processes files that contain non-English text such as Chinese. The package uses the efficient YUI minifier to minify JavaScript and CSS files that are all in English (that is, contain only ASCII characters). Because the YUI minifier doesn't handle non-ASCII characters (such as Chinese), the package automatically uses the JSMIN algorithm for files containing such characters. JSMIN is less efficient, but handles non-ASCII characters well.
     
  • Combines JavaScript files and CSS files. Loading a single large file is often much quicker than loading a series of small files, because that saves the overhead involved in all the request and response messages.

    If a CSS file contains image urls that are relative to the folder containing the CSS file itself, those urls are fixed up by the package. That way, they continue to work even if CSS files from several different folders are combined.
     
  • Allows you to configure the package so it only kicks in in release mode. That way, you see your individual files complete with white space and comments while developing, and reap the performance improvement in your live site.
     
  • Reduces the size of the HTML generated by your .aspx pages by removing white space and comments. Note that the .aspx files themselves are not affected, only the HTML sent to the browser.
     
  • Allows you to configure cookieless domains from which to load JavaScript files, CSS files and images. This way, the browser no longer sends cookies when it requests those files, reducing wait times for the visitor.
     
  • Lets you configure multiple cookieless domains. This causes the browser to load more JavaScript, CSS and image files in parallel.
     
  • Optimizes use of the browser cache by allowing the browser to store JavaScript, CSS and image files for up to a year. Uses version ids in file names to ensure the browser picks up new versions of your files right away, so visitor never see outdated files (details on how this works).
     
  • Unlike similar packages, doesn't add query strings when combining files or when inserting versions. This optimizes caching by proxy servers (many proxy servers won't cache files with query strings).
     
  • Converts image file names to lower case, to make it easier for those proxy and browser caches that do case sensitive file name comparisons to find your file in their caches - so they don't request the same file again.
     
  • Preloads images immediately when the page starts loading, instead of when the browser gets round to loading the image tags - so your images appear quicker. You can give specific images priority.
     
  • Helps you detect missing files by throwing an exception when a JavaScript file, CSS file or image is missing. By default, the package handles missing files silently, without throwing an exception.
     
  • To reduce CPU overhead and disk accesses caused by the package, it caches intermediate results, such as minified files. A cache entry is removed when the underlying file is changed, so you'll never serve outdated files.

Server compression

If you are interested in web site performance, you may be interested in this short digression into server compression.

IIS 6 and 7, and Apache as well, provide the option to gzip compress all text files (html, JavaScript, CSS, etc.) sent to the browser. All modern browsers know how to decompress those files. Compression can save you a lot of bandwidth and download time. It is not uncommon to reduce file sizes by way over 50%.

In IIS, compression is switched off by default for dynamic files, such as .aspx files. This is because it increases the load on the CPU. However, with the overabundence of CPU cycles on modern server hardware, switching on compression for dynamic files on your server is almost always a great idea. Also, IIS 6 and 7 allow you to set the compression level, so you can choose a level that you're comfortable with. Finally, IIS 7 can automatically switch off compression when CPU usage goes over a predetermined level (set by you), and switch it back on after CPU usage has dropped below a second level (also set by you). It even lets you cache compressed dynamic files, which makes compression extremely attractive.

Switching on basic compression on IIS 7 is easy, but getting the most out of it is a bit tricky. Switching on compression in IIS 6 is just tricky. Good places to find out more would be here (for IIS 7) and here (for IIS 6).

Or you could read chapter 10 of my book ASP.NET Site Performance Secrets where it is all spelt out in one place (believe me, this will save you a lot of time).

Installation

Installing the CombineAndMinify package involves these steps:

  1. Common installation - applies to all environments.
  2. IIS 7 related - only applies if your live site runs under IIS 7.
  3. IIS 6 related - only applies if your live site runs under IIS 6.
  4. Development environment related - only applies if you want to use the package not just in your live site, but also in your development environment.

Common Installation

  1. Compile the package:
     
    • Download the zip file with the source code, and unzip in a directory.
    • Open the CombineAndMinify.sln file in Visual Studio 2008 or later.
    • You'll find that the sources are organized in a solution, with these elements:

      1. Project CombineAndMinify is the actual package.
      2. Web site Testsite contains a lot of (functional but rather disorganised) test cases. Ignore this unless you want to test the package.
       
    • Compile the solution. This will produce a CombineAndMinify.dll file in the CombineAndMinify\bin folder.
  2. Update your web site:

    1. Add a reference to CombineAndMinify.dll to your web site (in Visual Studio, right click your web site, choose Add Reference)
    2. Add the custom section combineAndMinify to your web.config:

    <configuration>
    ...
    <configSections>
    ...
    <section name="combineAndMinify" type="CombineAndMinify.ConfigSection"/>
    ...
    </configSections>
    ...
    </configuration>
     
  3. Allow the package to process the head sections of your pages. That way, it can replace the tags loading individual JavaScript and CSS files with tags loading combined files:

    1. Make sure that the head tags of your pages have runat="server", like so:

    <head runat="server">

    When you create a new page in Visual Studio, it creates a head section with runat="server", so you probably don't have to change anything here.

    2. Add a folder called App_Browsers to the root folder of your web site.

    3. Use your favorite text editor (such as Notepad) to create a text file in the App_Browsers folder. Call it HeadAdapter.browser. Into that file, copy and paste this code:

    <browsers>
    <browser refID="Default">
    <controlAdapters>
    <adapter controlType="System.Web.UI.HtmlControls.HtmlHead"
    adapterType="CombineAndMinify.HeadAdapter" />
    </controlAdapters>
    </browser>
    </browsers>

    This tells ASP.NET to leave processing of all HtmlHead controls (which represent the head tags) to the class CombineAndMinify.HeadAdapter (which is part of the package).
     
  4. Make sure that your site has trust level Full. This allows it to access the combineAndMinify section. If your web.config contains

    <trust level=... />

    remove it. You'll than get the default trust level, which is Full.

IIS 7

Only do these steps if you use IIS 7 for your live site or your development environment. If you only use IIS 6 or IIS 7 in classic mode, skip to the IIS 6 section.

The package uses an HTTP Handler to combine and minify JavaScript and CSS files on the fly. To reduce CPU overhead, it caches the combined files, so they only get combined and minified after they have been changed on the file system.

To configure the HTTP Handler, add the following to your web.config:

</configuration>
...
<system.webServer>
<validation validateIntegratedModeConfiguration="false"/>
...
<handlers>
...
<add name="JavaScriptHandler" verb="*" path="*.js" type="CombineAndMinify.HttpHandler, CombineAndMinify" resourceType="Unspecified"/>
<add name="CssHandler" verb="*" path="*.css" type="CombineAndMinify.HttpHandler, CombineAndMinify" resourceType="Unspecified"/>
</handlers>
...
</system.webServer>
...
</configuration>

IIS 6

Only do these steps if you use IIS 6 or IIS 7 in classic mode. If you use IIS 7, do the installation for IIS 7. If you want to invoke the package not only in your live environment but your development environment as well, than do the installation related to your development environment as well.

1. Configure the HTTP Handler in your web.config:

<configuration>
...
<system.web>
...
<httpHandlers>
...
<add verb="*" path="*.js" type="CombineAndMinify.HttpHandler, CombineAndMinify" />
<add verb="*" path="*.css" type="CombineAndMinify.HttpHandler, CombineAndMinify" />
</httpHandlers>
...
</system.web>
...
</configuration>

2. Send all requests for JavaScript and CSS files to the ASP.NET handler. That allows the package to combine and minify them.

  • Open the IIS Manager - click Start | Administrative Tools | Internet Information Services (IIS) Manager.
  • Expand your server. Expand Web Sites. Right click your web site and choose Properties.
  • Click the Home Directory tab, and then click the Configuration button.
  • Get the path to the ASP.NET handler:

    1. Click the line with the .aspx extension.
    2. Click the Edit button.
    3. Copy the contents of the Executable box. If you use .Net 2 or .Net 3.5, it will be C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\aspnet_isapi.dll.

    4. Click Cancel to dismiss the dialog.
    5. Tell IIS 6 to let ASP.NET handle all requests for files with extension .js:
     
    • Click the Add button.
    • Paste the path to the ASP.NET handler you just found into the Executable field.
    • Enter .js into the Extension field.
    • Uncheck the Verify that file exists box.
    • Click OK.
    • Repeat the last step, but now for extension .css.
    • Click OK to dismiss the Configuration dialog.
    • Click the Apply button, and then click OK to dismiss the Properties dialog.

Development Environment

By default, the package is only active when your site is in Release mode - that is, if you have

<compilation debug="false">

in your web.config. That makes sense, because the package doesn't affect the functionality of your site - it just makes it faster. If this arrangement works for you, there is no need to do any installation related to your development environment, so you can skip this section.

If you do want to activate the package in your development environment, you need to run your debugging sessions under IIS 7, instead of under Cassini (the web server that normally runs when you hit F5 in Visual Studio). This is because the package uses an HTTP Handler to process JavaScript and CSS files, and Cassini in .Net 3.5 doesn't support HTTP Handlers.

Note that IIS 7 is included in the Home Premium, Business, Enterprise and Ultimate versions of Windows Vista and Win 7 (but not the Starter and Home versions). That means you probably already have it on your system.

Here is how to run your debugging sessions under IIS 7:

  1. Install IIS 7 on your development machine
  2. Create a development site in IIS 7
  3. Get Visual Studio to use IIS 7 instead of Cassini
  4. Do the installation steps related to IIS 7
  5. Make sure the package is active in debug mode

Let's go through these steps one by one.

1. Install IIS 7 on your development machine

  • Click Start | Control Panel | Programs | Turn windows features on or off.
  • Check Internet Information Services.
  • Expand the Internet Information Services node, expand Web Management Tools and make sure IIS Management Console is checked. This will allow you to run the IIS Manager.
  • Expand IIS 6 Management Compatibility and make sure IIS Metabase and IIS 6 configuration compatibility is checked. You need this to be able to use IIS from Visual Studio.
  • Expand the World Wide Web Services node and then the Application Development Features node. Make sure that ASP.NET is checked, so the server can run ASP.NET applications.
  • Also under World Wide Web Services, expand Security and make sure Windows Authentication is selected. You need this to be able to use IIS from Visual Studio.
  • Click OK. Windows will take a while to implement the changes.

2. Create a development site in IIS

Now that you have installed IIS 7 on your machine, create a site in IIS 7 that Visual Studio can use for debugging sessions:

  • While still in IIS Manager, expand your machine. Right click Sites and choose Add Web Site. The Add Web Site dialog opens.
  • Make up a site name. As the physical path, enter the root folder of your source files – the one with default.aspx and web.config. Enter a port that isn't already in use, for example 1080. Click OK.
  • Enable Windows Authentication. Double click your new site, double click Authentication, right click Windows Authentication and choose Enable.

3. Get Visual Studio to use IIS 7 instead of Cassini

If you use a Web Site:

  • Run Visual Studio as administrator. Right click your web site and choose Start Options. The Property Pages window opens, with the Start Options tab selected.
  • In the Server section, select Use custom server. In the Base Url field, enter the localhost address with the port you entered when you created the development site in IIS manager - for example http://localhost:1080.
  • Click OK.

If you use a Web Application:

  1. In Visual Studio, right click the web application and choose Properties. The properties window opens.
  2. Click the Web tab.
  3. Scroll down to the Servers section and select Use Local IIS Web server. In the Project Url field, enter the localhost address with the port you entered when you created the development site in IIS manager.
  4. Save the properties.

Now when you hit F5 in Visual Studio to run your site in debug mode, you'll find it runs under IIS 7 instead of under Cassini. Apart from that, your debugging experience is the same - you can still set breakpoints, etc.

4. Do the installation steps related to IIS 7

Now that you're using IIS 7 to do development, do the IIS 7 related installation if you have not already done that.

5. Make sure the package is active in debug mode

Follow the steps listed here.
Configuration

By default, the package minifies and combines JavaScript and CSS files. To use the other features, or to switch off minification or combining of JavaScript or CSS files, add a combineAndMinify element to your web.config file, like so:

<configuration>
...
<combineAndMinify ... >

</combineAndMinify>
...
</configuration>

The combineAndMinify element supports these attributes and child elements:

  • active
  • combineCSSFiles
  • combineJavaScriptFiles
  • minifyCSS
  • minifyJavaScript
  • cookielessDomains
  • enableCookielessDomains
  • preloadAllImages
  • prioritizedImages
  • makeImageUrlsLowercase
  • insertVersionIdInImageUrls
  • exceptionOnMissingFile
  • removeWhitespace
  • headCaching

active

Determines when the package is active. When it is not active, it doesn't affect your site at all and none of the other attributes or child elements listed in this section have any effect.

Value Description
Never Package is never active, irrespective of debug mode
Always Package is always active, irrespective of debug mode
ReleaseModeOnly (default) Package is only active in release mode
DebugModeOnly Package is only active in debug mode

Example

<configuration>
...
<combineAndMinify active="Always" >

</combineAndMinify>
...
</configuration>

Whether your site is in debug or release mode depends on the debug attribute of the compilation element in your web.config file. If that attribute is set to false, your site is in release mode (as it should be when live). When it is set true, it is in debug mode (as it should be in development). It looks like this in web.config:

<configuration>
...
<system.web>
<compilation debug="true">
...
</compilation>
...
</system.web>
...
</configuration>

Note that by default, the package is only active in release mode - so you won't see any effect while you're developing. To ensure that the package is active in both release mode and in debug mode as well, set active to Always, as shown in the example above.

Note that the active attribute acts as a master switch for the whole package. It's like the ignition in a car - if you don't turn on the ignition (or at least the battery), pressing any other buttons on the dashboard won't do anything.

combineCSSFiles

Determines whether the package combines CSS files.

Value Description
None CSS files are never combined
PerGroup (default) CSS files are combined per group. See explanation below.
All All CSS files are combined into a single CSS file.

Example

<configuration>
...
<combineAndMinify combineCSSFiles="All" >

</combineAndMinify>
...
</configuration>

To see what is meant with "per group", have a look at this example:

<link rel="Stylesheet" type="text/css" href="/css/site1.css" />
<link rel="Stylesheet" type="text/css" href="/css/site2.css" />

<script type="text/javascript" src="js/script1.js"></script>

<link rel="Stylesheet" type="text/css" href="/css/site3.css" />
<link rel="Stylesheet" type="text/css" href="/css/site4.css" />

By default, the package tries to ensure that the order of the JavaScript and CSS definitions doesn't change when JavaScript and CSS files are combined. It does this by grouping CSS files whose tags are after each other. In this case, there are 2 groups - site1.css and site2.css, and site3.css and site4.css. This causes the browser to load the CSS and JavaScrip definitions in the exact same order as when the files had not been combined:

  1. Combined file with all CSS definitions in site1.css and site2.css
  2. script1.js
  3. Combined file with all CSS definitions in site3.css and site4.css

You get this behaviour when you set combineCSSFiles to PerGroup (which is the default). The package supports grouping for JavaScript files as well, which is controlled by the combineJavaScriptFiles attribute.

Combining all CSS files into one file

If you set combineCSSFiles to All, all CSS files get combined into a single file. The link tag to load that combined CSS file gets placed where the link tag of the first CSS file used to be. In our example, that causes this load order:

  1. Combined file with all CSS definitions in site1.css, site2.css, site3.css and site4.css
  2. script1.js

As you see, the CSS definitions in site3.css and site4.css now get loaded before script1.js, instead of after script1.js. As a result, the number of CSS files that need to be loaded is reduced from two to one (as compared with grouping), but you also change the order in which CSS and JavaScript definitions are loaded. It depends on your site whether that is an issue or not.

Loading CSS files from another site

If you decide to set combineCSSFiles to All, be sure that your site doesn't load CSS files from another web site. This is uncommon, but if yours is one of the rare sites that does this, consider this example:

<link rel="Stylesheet" type="text/css" href="/css/site1.css" />
<link rel="Stylesheet" type="text/css" href="/css/site2.css" />

<link rel="Stylesheet" type="text/css" href="http://anothersite.com/css/other.css" />

<link rel="Stylesheet" type="text/css" href="/css/site3.css" />
<link rel="Stylesheet" type="text/css" href="/css/site4.css" />

The package never combines CSS files from other sites - not with CSS files from your site, not with CSS files from the other site. As a result, if you change combineCSSFiles to All, the package will combine all CSS files loaded from your site (but not those from the other site), causing the following load order:

  1. Combined file with all CSS definitions in site1.css, site2.css, site3.css and site4.css
  2. http://anothersite.com/css/other.css

The definitions in other.css now came after those in site3.css and site4.css, meaning they may take precedence - which could break your CSS.

combineJavaScriptFiles

Determines whether the package combines JavaScript files.

Value Description
None JavaScript files are never combined
PerGroup (default) JavaScript files are combined per group. See explanation below.
All All JavaScript files are combined into a single JavaScript file.

Example

<configuration>
...
<combineAndMinify combineJavaScriptFiles="All" >

</combineAndMinify>
...
</configuration>

As you saw in the description of combineCSSFiles, the package groups JavaScript files in the same way as CSS files. Similarly, if you set combineJavaScriptFiles to All, all JavaScript files that are loaded from your site get combined into a single JavaScript file.

However, there is one major difference with combining CSS files: When you combine all JavaScript files into a single file (combineJavaScriptFiles is All), the script tag for the combined file winds up at the location where the last original script tag used to be. This in contract to a combined CSS file, which winds up at the location of the first original CSS tag.

This makes life easier if you load JavaScript libraries from extenal sites. If you use a popular package such as jQuery, you can load it from the free Google Content Delivery Network (CDN) and also from the free Microsoft CDN - which saves you bandwidth and download time (details about those CDNs are in chapter 13 of my book).

Take this example:

<script type="text/javascript" src="/js/script1.js" ></script>
<script type="text/javascript" src="/js/script2.js" ></script>

<!-- load jQuery from free Google CDN -->
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>

<script type="text/javascript" src="/js/script3.js" ></script>
<script type="text/javascript" src="/js/script4.js" ></script>

script3.js and script4.js may well be dependent on jquery.min.js. If combineJavaScriptFiles is set to PerGroup, you wind up with this load order:

  1. Combined file with all JavaScript definitions in script1.js and script2.js
  2. jquery.min.js
  3. Combined file with all JavaScript definitions in script3.js and script4.js

So all definitions load in the same order as before the files were combined, as you'd expect. Now when you set combineJavaScriptFiles to All, you wind up with this load order:

  1. jquery.min.js
  2. Combined file with all CSS definitions in script1.js, script2.js, script3.js and script4.js

This would probably still work well, because the definitions in script3.js and script4.js still get loaded after those in jquery.min.js. script1.js and script2.js now get loaded after jquery.min.js, but than jquery.min.js wouldn't have been dependent on those files anyway.

minifyCSS

Determines whether the package minifies CSS files.

Value Description
true(default) CSS files get minified
false CSS files do not get minified

Example

<configuration>
...
<combineAndMinify minifyCSS="false" >

</combineAndMinify>
...
</configuration>

Minifying a file means removing redundant white space and comments. This not only reduces your bandwidth usage and download times, but also makes it harder for outsiders to reverse engineer your web site. It also encourages developers to add comments to their CSS files (and especially their JavaScript files), now that those comments do not travel over the wire to the browser.

It is normally safe to minify files, so this feature is enabled by default. However, just in case removing white space or comments breaks your file, the option exists to switch it off.

Remember that the package doesn't minify your source files. It reads your source files, and minifies their content before sending them to the browser. To save CPU cycles, it caches the minified versions, using file dependencies to ensure cached versions are removed the moment you change the underlying files.

minifyJavaScript

Determines whether the package minifies JavaScript files.

Value Description
true(default) JavaScript files get minified
false JavaScript files do not get minified

Example

<configuration>
...
<combineAndMinify minifyJavaScript="false" >

</combineAndMinify>
...
</configuration>

cookielessDomains

To have your JavaScript, CSS and image files loaded from one or more cookieless domains, specify those domains using a cookielessDomains child element.

Example

<configuration>
...
<combineAndMinify ... >
<cookielessDomains>
<add domain="http://static1.mydomain.com"/>
<add domain="http://static2.mydomain.com/"/>
</cookielessDomains>
</combineAndMinify>
...
</configuration>

Cookieless domains help you boost web site performance in two ways. They let you cut down on the overhead created by cookies. And they let you get the browser to load more JavaScript, CSS and image files in parallel. Let's look at the cookie overhead first, and then the parallel loading.
Cookie overhead

If your site sets a cookie on the browser, then each time the browser sends a request to your site, that request contains the cookie. The issue is that the cookie is not only sent when requesting an .aspx file, but also when requesting static files, such JavaScript files, CSS files and images. In most cases, this is a waste of bandwidth (the exception would be if you have a handler that processes for example JavaScript files and that uses the cookie).

However, the browser won't send the cookie to another domain or sub domain. So if your page is at http://www.mydomain.com/page.aspx (using subdomain www), and you put your images and other static files on http://static1.mydomain.com (using subdomain static1), than the browser won't send cookies when requesting static files.

As an aside, if your site uses cookies (or ASP.NET sessions, which uses cookies), you should never allow visitors to access your pages without a subdomain. That is, don't allow them to access http://mydomain.com/page.aspx. Otherwise, if a visitor first accesses http://www.mydomain.com/page.aspx (using subdomain www), sets a cookie, and then comes back via http://mydomain.com/page.aspx (no subdomain), the browser won't send the cookie! IIS 7 makes it very easy to redirect requests to http://mydomain.com to http://www.mydomain.com using an entry in web.config. See Microsoft's iis.net, or the image control adapter included in chapter 12 of my book ASP.NET Site Performance Secrets.
Parallel loading

When a browser loads a page, it loads the static files (images, JavaScript files, CSS files) in parallel to reduce the visitor's wait time. However, the browser limits the number of files that are loaded in parallel. Modern browsers (IE7 and better, Firefox, Chrome, Safari) have a limit of about 6, while older browsers (such as IE6) have a limit of 2.

However, this limit is per (sub)domain. It isn't per IP address. This means that if you spread your static files over for example 2 cookieless domains, you allow the browser to load up to two times more files in parallel.

Using cookieless domains on your site

If you add a cookielessDomains element with one or more domains to the combineAndMinify element, the package adds those domains to the urls of all static files in your site. This includes images referenced from CSS files.

This means that if you use:

<configuration>
...
<combineAndMinify ... >
<cookielessDomains>
<add domain="http://static1.mydomain.com"/>
</cookielessDomains>
</combineAndMinify>
...
</configuration>

Then for example

<img src="images/ball3.png" height="10" width="10" />

is replaced by

<img src="http://static1.mydomain.com/images/ball3.png" height="10" width="10" />

If you define multiple domains, such as:

<configuration>
...
<combineAndMinify ... >
<cookielessDomains>
<add domain="http://static1.mydomain.com"/>
<add domain="http://static2.mydomain.com"/>
</cookielessDomains>
</combineAndMinify>
...
</configuration>

Then the package attempts to spread the files over the available domains (note that you can add more than 2 domains if you want). This to get the browser to load more files in parallel. So if you have these image tags:

<img src="images/ball3.png" />
<img src="images/woodentoy4.png" />

You would wind up with:

<img src="http://static2.mydomain.com/images/ball3.png" />
<img src="http://static1.mydomain.com/images/woodentoy4.png" />

To create the subdomains, log into your account at your domain registrar and create the static1, static2, etc. subdomains (you can use any subdomain names you like). Make sure they point to the same IP address as your www subdomain. This way, you don't have to physically move your static files. Note that every subdomain that is not your www subdomain acts as a "cookieless" subdomain - it isn't like there are special "cookieless" subdomains as such.

Contrary to what you may think, if you have say 2 cookieless domains, the package won't use one domain for half the static files and the other domain for the other half. This is because it needs to make sure that on every page, a given static file is always given the same domain. If images/ball3.png were turned into http://static1.mydomain.com/images/ball3.png on one page, but to http://static2.mydomain.com/images/ball3.png on a second page, than the browser won't find ball3.png in its cache when it hits the second page, even if it stored ball3.png when it accessed the first page. Because of the different domains, it would regard the two urls as different, even though they actually point at the same resource.

Because of this requirement, the package uses the hash code of the file name to work out which domain to use. So if there are two domains, than if the hash is even it uses the first domain, and if it is odd it uses the second domain. Because it is unlikely that 50% of file names have an even hash code, you are unlikely to get a perfect distribution of the static files over the available domains.

enableCookielessDomains

Determines whether cookieless domains are used.

Value Description
Never Cookieless domains are never used.
Always(default) Cookieless domains are always used, provided that 1) the package is active, and 2) you have defined a cookielessDomains element with cookieless domains.
ReleaseModeOnly Cookieless domains are only used in release mode.
DebugModeOnly Cookieless domains are only used in debug mode.

Example

<configuration>
...
<combineAndMinify active="Always" enableCookielessDomains="ReleaseModeOnly" >
<cookielessDomains>
<add domain="http://static1.mydomain.com"/>
<add domain="http://static2.mydomain.com"/>
</cookielessDomains>
</combineAndMinify>
...
</configuration>

This option is really only useful if you decide to activate the package in your development environment. In that case, you may decide to only use cookieless domains in release mode, while using all the other features of the package in both release and debug mode.

The reason for this is that if you have new images in your development environment that are not yet on your live site, than they won't show up in your development environment if you use the cookieless domains - which point to your live site.

If you want to take that route, set enableCookielessDomains to ReleaseModeOnly.

The default value for enableCookielessDomains is Always. However, keep in mind that for the package to use cookieless domains, it has to be active. And by default, it is only active in release mode.

preloadAllImages

Determines whether the package inserts code that preloads all images when the page starts loading.

Value Description
true All images are preloaded
false(default) No images are preloaded (except for those specified using child element prioritizedImages, described further below)

Example

<configuration>
...
<combineAndMinify preloadAllImages="true" >

</combineAndMinify>
...
</configuration>

Normally, the browser only starts loading an image after it has encountered its image tag in the HTML or in a CSS file. If the image tag is towards the end of a big page, or if it takes a while to load the CSS file, it can take a while before image loading starts.

To get the browser to start loading all images immediately when the page itself starts loading, set the preloadAllImages attribute to true. The package than generates JavaScript at the start of the page head to load each image into the browser cache, along these lines:

<script type="text/javascript">
var img0=new Image();img0.src='http://www.codeproject.com/images/ball3.png';
var img1=new Image();img1.src='http://www.codeproject.com/css/chemistry.png';
var img2=new Image();img2.src='http://www.codeproject.com/images/toy4.png';
...
</script>

Now when the browser encounters an image tag, the image is already in browser cache, so the browser can show the image right away.

prioritizedImages

Allows you to prioritize certain images for preloading.

Example

<combineAndMinify ... >
<prioritizedImages>
<add url="images/salesbutton.png"/>
<add url="images/logo.png"/>
</prioritizedImages>
</combineAndMinify>

If you have many images on your pages, you may want to prioritize certain images. For example, if your "order now" button is an image, you want that image in front of your visitors as soon as possible.

You can use prioritizedImages without setting preloadAllImages to true. Here is how these two attributes interact:

prioritizedImages preloadAllImages Images preloaded
Empty or not present true All the images referred to from CSS files and all the images on the page are preloaded. They are loaded in the order in which their tags appear in the CSS or in the HTML. Images referred to from CSS are preloaded before images on the page itself.
Empty or not present false None
One or more urls true All urls listed in prioritizedImages are preloaded first, than all the other images.
One or more urls false Only the urls listed in prioritizedImages are preloaded

makeImageUrlsLowercase

Determines whether the package makes all image urls lowercase.

Value Description
true All images urls are converted to lowercase
false(default) Images url casing is left as it is

Example

<configuration>
...
<combineAndMinify makeImageUrlsLowercase="true" >

</combineAndMinify>
...
</configuration>

You may be using inconsistent casing in your web pages to refer to the same image. For example:

<img src="/images/woodentoy4.png" height="10" width="10" />
...
<img src="/images/WoodenToy4.png" height="10" width="10" />

Assume a browser or proxy loads woodentoy4.png and stores it in its cache. When it then needs to load WoodenToy4.png, it may not recognize it is the same as the woodentoy4.png that it already has in cache, and send a superfluous request for WoodenToy4.png.

To prevent this, set makeImageUrlsLowercase to true. This way, all images urls in the HTML and CSS sent to the browser will be lowercase, so there is no inconsistent casing. Note that the package doesn't change your source files. Instead, it changes the HTML that gets generated from your source files and sent to the browser.

insertVersionIdInImageUrls

Determines whether the package 1) inserts version ids in image file names and 2) allows the browser to cache images for up to a year.

Value Description
true Version ids are inserted in image file names. Requires changes to web.config (see below).
false(default) No version ids are inserted in image file names

Example

<configuration>
...
<combineAndMinify insertVersionIdInImageUrls="true" >

</combineAndMinify>
...
</configuration>

When a browser receives an image, it stores it in its browser cache. That way, if it needs to load the image again, it may still be in cache, so there is no need to send a request to the server. The result is less waiting for the visitor and less bandwidth used by your server.

One issue here is how long the browser should keep the image in its cache. Too long, and you may have visitors looking at outdated images. Too short, and the browser sends more requests for the image than necessary.

By setting insertVersionIdInImageUrls to true, you get the best of both worlds:

  • It causes the package to send HTTP Response Headers when sending images that tell the browser it can cache the image for up to a year - the maximum you can ask for according to the HTTP specification.
  • And it inserts a version id into the image file name as used in image tags (both in the page HTML and in the CSS). The package calculates that version id from the last update time of the image file - so if you update an image, the version id changes. That way, when you update an image, the browser immediately picks up the new image. It won't pick up the image it has in cache, because that has a file name with the old version id.

A few more details about how this feature works:

  • The package only inserts the version id in the tags that are sent to the browser. The file names of your images on disk stay the same.
  • There is no need to change your image file names manually. When the browser sends a request for an image, the image file name will have the version id. But the package takes care of stripping out the version id and serving the correct image to the browser.
  • To find out the version id, the package needs to access the file system to find out the last update time of the image file. You don't want this to happen for each request, so the package stores the version ids in server cache. These cache entries are invalidated the moment the underlying file changes, so the cache is never outdated.
  • Why insert the version id in the file name? Why not just add it as a query string? That would be a bit easier to handle. However, proxies (intermediate servers that pass on messages on the Internet) are less likely to cache files with query strings. So by inserting the version id in the file name instead of using a query string, you gain maximum proxy caching.
  • There is no counterpart of insertVersionIdInImageUrls for JavaScript and CSS files, because the package always uses version ids and long term caching for those files.

Additional Installation

Now that the package inserts a version id in the image file names used in image tags, the browser will send requests for file names with version ids. Those file names won't match actual image files on your server, because they don't have the version ids. To solve this, you need to get the package to handle incoming requests for images, so it can remove the version id and load the correct file.

Additional Installation for IIS 7

Take these steps if you use IIS 7 for your live site, and also if you use the package in your development environment. The steps to take for IIS 6 are here.

In the installation instructions for IIS 7 for the entire package, you modified your web.config file to get the HTTP Handler to handle .js and .css files.

Now change web.config again, so the HTTP Hander also handles .gif, .png and .jpg files:

</configuration>
...
<system.webServer>
<validation validateIntegratedModeConfiguration="false"/>
...
<handlers>
...
<!-- required for insertVersionIdInImageUrls attribute -->
<add name="GifHandler" verb="*" path="*.gif" type="CombineAndMinify.HttpHandler, CombineAndMinify" resourceType="Unspecified"/>
<add name="PngHandler" verb="*" path="*.png" type="CombineAndMinify.HttpHandler, CombineAndMinify" resourceType="Unspecified"/>
<add name="JpegHandler" verb="*" path="*.jpg" type="CombineAndMinify.HttpHandler, CombineAndMinify" resourceType="Unspecified"/>

<!-- required for all features -->
<add name="JavaScriptHandler" verb="*" path="*.js" type="CombineAndMinify.HttpHandler, CombineAndMinify" resourceType="Unspecified"/>
<add name="CssHandler" verb="*" path="*.css" type="CombineAndMinify.HttpHandler, CombineAndMinify" resourceType="Unspecified"/>
</handlers>
...
</system.webServer>
...
</configuration>

Additional Installation for IIS 6 or IIS 7 in classic mode

Follow these steps if your live site uses IIS 6. If your live site uses IIS 7 or higher, you can skip this.

1. In the installation instructions for IIS 6 for the entire package, you modified your web.config file to get the HTTP Handler to handle .js and .css files.

Now change web.config again, so the HTTP Hander also handles .gif, .png and .jpg files:

<configuration>
...
</system.web>
...
<httpHandlers>
...
<!-- required for insertVersionIdInImageUrls attribute -->
<add verb="*" path="*.gif" type="CombineAndMinify.HttpHandler, CombineAndMinify" />
<add verb="*" path="*.png" type="CombineAndMinify.HttpHandler, CombineAndMinify" />
<add verb="*" path="*.jpg" type="CombineAndMinify.HttpHandler, CombineAndMinify" />

<!-- required for all features -->
<add verb="*" path="*.js" type="CombineAndMinify.HttpHandler, CombineAndMinify" />
<add verb="*" path="*.css" type="CombineAndMinify.HttpHandler, CombineAndMinify" />
</httpHandlers>
...
</system.web>
...
<configuration>

2. In the installation instructions for IIS 6, you also saw how to route all requests for .js and .css files to ASP.NET. Now use the same instructions to do the same for these extensions:

  • .gif
  • .png
  • .jpg

For a lot more on browser caching and proxy caching, see www.iis.net or chapter 12 of my my recently released book ASP.NET Performance Secrets.

exceptionOnMissingFile

Determines whether the package throws an exception when an image file is missing.

Value Description
Never
(default)
The package never throws an exception when an image file is missing.
Always The package always throws an exception when an image file is missing.
ReleaseModeOnly The package only throws an exception if the site is in release mode.
DebugModeOnly The package only throws an exception if the site is in debug mode.

Example

<configuration>
...
<combineAndMinify exceptionOnMissingFile="DebugModeOnly" insertVersionIdInImageUrls="true" >

</combineAndMinify>
...
</configuration>

Assume insertVersionIdInImageUrls is set to true, so the package inserts a version id in all image urls. This means it has to access each image file to find its last updated time. What happens if the image file cannot be found? That is determined by the exceptionOnMissingFile attribute:

  • If exceptionOnMissingFile is active (see table above) and the package finds that an image file cannot be found, it throws an exception with the path of the image. That makes it easier to find missing images.
  • If exceptionOnMissingFile is not active, the package doesn't throw an exception but recovers by not inserting a version id in the image url.

If all images should be present in your development environment, than it makes sense to set exceptionOnMissingFile to DebugModeOnly. That way, you quickly find broken images while developing your site, while preventing exceptions in your live site where you probably prefer a broken image over an exception.

What about JavaScript and CSS files? The package accesses these files when combining and / or minifying them:

  • If exceptionOnMissingFile is active and a JavaScript or CSS files can't be found, you'll get an exception, just as with images.
  • If exceptionOnMissingFile is not active and a JavaScript or CSS files can't be found, it just writes a comment in the combined and / or minified file, specifying the full name of the file that couldn't be found.

Keep in mind that if you want exceptions while the site is in debug mode, you have to ensure that the package is actually active in debug mode - set active to Always to make that happen.

removeWhitespace

Determines whether the package removes superfluous white space and comments from the HTML of the page.

Value Description
true Superfluous white space and comments are removed.
false
(default)
No superfluous white space and comments are removed.

Example

<configuration>
...
<combineAndMinify removeWhitespace="true" >

</combineAndMinify>
...
</configuration>

When you set removeWhitespace to true, the package removes all HTML comments from the page and collapses all runs of white space into a space. However, if a run of white space contains one or more line breaks, it is collapsed into a line break. That way, inline JavaScript will not be broken.

headCaching

Determines how tags for combined JavaScript files and CSS files are cached.

Value Description
None
(default)
Caching of replacement tags is switched off.
PerSite There is a single cache entry for the entire site.
PerFolder There is a cache entry per folder.
PerPage There is a cache entry per page (ignoring any query string).
PerUrl There is a cache entry per url (including any query string).

Example

<configuration>
...
<combineAndMinify headCaching="PerSite" >

</combineAndMinify>
...
</configuration>

Even though the package caches all minified and combined files, there is still some work involved in replacing tags of individual JavaScript files and CSS files with tags of combined files. Without further caching, this needs to be done for each page request.

To reduce the CPU usage involved in this, the package provides the option to cache the replacement tags. The recommended way to do this depends on the way you load your JavaScript and and CSS files:

Situation Recommended
Value
The additional CPU usage of the package is not an issue. Or the tags to load JavaScript and CSS files are totally ad hoc per page. None
(default)
All pages load the same JavaScript and CSS files in the same order. For example, all pages uses a single master page, and the master page has all the script and link tags to load the JavaScript and CSS files. PerSite
Your pages are split over folders, and the JavaScript and CSS files you load depend on the folder. For example, pages in the admin folder all load the same JavaScript and CSS files in the same order, but those files are different from the ones loaded by pages in the products folder. PerFolder
Each page loads different JavaScript and / or CSS files. However, the query string doesn't influence which files are loaded. So toys.aspx?id=1 and toys.aspx?id=2 load the same files in the same order, but categories.aspx loads different files. PerPage
The JavaScript and CSS files used by a page depend on the entire url, including its query string. So toys.aspx?id=1 and toys.aspx?id=2 load different files. PerUrl

The headCaching attribute really comes into its own if you load JavaScript and CSS files from a master page or a shared user control. This is because the package caches entire groups of tags, and the tags to replace those groups with. This process is sensitive to for example white space in between tags. That means that

<script type="text/javascript" src="/js/script1.js" ></script>
<script type="text/javascript" src="/js/script2.js" ></script>

and

<script type="text/javascript" src="/js/script1.js" ></script>

<script type="text/javascript" src="/js/script2.js" ></script>

are not the same, due to the extra white line in the second block.

Conclusion

This package will improve the performance of any web site that loads JavaScript, CSS or image files. Please give it a try with your site. If you find any bugs or have any trouble with the documentation, let me know and I'll try to fix the issue. Feature requests would be welcome too.

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