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Compressed JavaScript Compression

In this article, we extrapolate our favorite CSS-compression technique for JavaScript. Below, we outline the steps required to auto-compress your JavaScript documents via gzip and PHP. Two different compression methods are presented. The first method does not require htaccess, but rather involves the manual editing of JavaScript files. The second method employs htaccess to do all the work for you, thus requiring much less effort to implement. In either case, the result is the same: automatically compressed content delivered only to supportive browsers, resulting in reduced bandwidth, faster loading times, and smiling visitors :)

Method One

Overview: This method involves adding a small PHP script to your JavaScript document and replacing its .js extension with a .php extension.

Place the following PHP script into the top of any JavaScript document(s) that you wish to compress. Then change the .js extension to .php, to arrive at something similar to compressed-js.php. Remember to use the new name when referencing the file (i.e., <script type="text/javascript" src="compressed-js.php"></script>):

   ob_start ("ob_gzhandler");
   header ("content-type: text/javascript; charset: UTF-8");
   header ("cache-control: must-revalidate");
   $offset = 60 * 60;
   $expire = "expires: " . gmdate ("D, d M Y H:i:s", time() + $offset) . " GMT";
   header ($expire);

Here is the same PHP script commented with functional explanations:


   // initialize ob_gzhandler function to send and compress data
   ob_start ("ob_gzhandler");

   // send the requisite header information and character set
   header ("content-type: text/javascript; charset: UTF-8");

   // check cached credentials and reprocess accordingly
   header ("cache-control: must-revalidate");

   // set variable for duration of cached content
   $offset = 60 * 60;

   // set variable specifying format of expiration header
   $expire = "expires: " . gmdate ("D, d M Y H:i:s", time() + $offset) . " GMT";

   // send cache expiration header to the client broswer
   header ($expire);


Functional Summary: The previous PHP function will first check to see if the browser requesting the file will accept "gzip-deflate" encoding. If no such support is detected, the requested file is sent without compression. Next, the function sends a header for the content type and character set (in this case, "text/javascript" and "UTF-8"). Then, a "must-revalidate" "cache-control" header requires revalidation against currently specified variables. Finally, an "expires" header specifies the time duration for which the cached content should persist (one hour in this case).

Method Two

Overview: This method involves placing the PHP script in a separate .php file and adding a set of rules to an .htaccess file.

A more discrete, unobtrusive method for compressing JavaScript involves two steps. First, save the script provided in the first method (above) as a seperate gzip-js.php file and place it in a JavaScript-exclusive directory. Then, add the following ruleset to an .htaccess file located in the same JavaScript-exclusive directory (i.e., the JavaScript directory should contain only .js files):

# JavaScript compression htaccess ruleset
AddHandler application/x-httpd-php .js
php_value auto_prepend_file gzip-js.php
php_flag zlib.output_compression On

Here is the same .htaccess ruleset commented with functional explanations:

# JavaScript compression htaccess ruleset

# process all JavaScript files in current directory as PHP
AddHandler application/x-httpd-php .js

# prepend the PHP script to all PHP files in the current directory
php_value auto_prepend_file gzip-js.php

# compress all parsed PHP pages from current directory
# this rule is redundantly present as the first line of the PHP script
php_flag zlib.output_compression On

Functional Summary: The .htaccess rules above first instruct Apache to parse all JavaScript files in the current directory as PHP. After this, Apache is instructed to insert the contents of the “gzip-js.php” file into the beginning of each PHP file (i.e., files with .js extensions renamed to .php) parsed from the current directory. And finally, Apache is instructed to compress automatically every parsed document in the current directory.

Confirmed Browsers

  • Internet Explorer 5 and up: works great
  • Netscape Navigator 6 and up: works fine
  • Mozilla/Firefox: all versions seem to work
  • Opera: does not cache compressed content

See also: Compress CSS and JavaScript with PHP at WP-Mix.


About the Author
Jeff Starr = Designer. Developer. Producer. Writer. Editor. Etc.
WP Themes In Depth: Build and sell awesome WordPress themes.

29 responses to “Compressed JavaScript Compression”

  1. It seems that when using the first method, on FF Mac, the javascript and css files are compressed, but not cached.

    Am I wrong ?

  2. Perishable 2007/05/15 2:31 pm

    As far as I know, Firefox will cache gzipped content delivered via either method, however Opera will not, regardless of method.

  3. I use FireBug to see what’s cached and what’s not, and my style.php is not (according to the color of the bar).

    I may be wrong but anyway, thank you for your fast answer, and also, thank you for that great blog you write.

  4. Perishable 2007/05/16 9:16 am

    I am looking into this.. some questions for you:

    1. You mention that the first method does not work – have you tried the second method?

    2. You mention that your "style.php" is not being cached – are you sure you have properly implemented the method (e.g., correct file(s), name(s), etc.) – this post describes JavaScript compression – "style.php" sounds like it may contain CSS content?

    3. Does Firefox cache uncompressed content from your site?

    4. What method(s) are you using to send the proper caching headers for your content?

    I also appreciate the information from your second comment. Some questions for you:

    1. I have just finished testing the issues you mention and could not find a problem – did you happen to clear your cookies in between sessions (for the "remember me" dilemma)?

    2. The WP Admin shows that you are indeed subscribed to comments on this post – are you not receiving the email notifications?

    3. You mention sk2 plugin – if there were a problem, how would sk2 be involved? do you know of any potential conflicts between parties?

    I find your concerns very important. Thank you for helping me look further into these issues.

  5. Note: I think your sk2 plugin make the

    “Save information for future comments”

    and the

    “Notify me of followup comments via e-mail”

    checkboxes stop working.

  6. You are really kind taking me in such consideration… I appreciate it a lot !

    1. The second method doesn’t work because of my hosting (1000GP mutualised at OVH)

    2. I’ve posted in this post by mistake, but be sure i used the css method in the stylesheet, and the js method in my javascript file.

    3. Firefox cache the files uncompressed (.css, .js), but don’t cache the compressed one (.php).

    4. I use exactely what you advice, just changed the cache-duration to 3600*24*7.

    1. No, i never clear cookies manualy.

    2. The cookies has been “sent to me”, on my second comment (the ones in a row).

    3. I think the captcha that appeared before I had cookies broke the process of the WP comment system.

    -> When I first commented on this post, I checked the boxes, then clicked “submit”. I was redirected to a blank page with a captcha. When I submitted the captcha, my comment was posted, but I think the process of storing cookie on my computer was broken

    I hope it will help you :)

  7. Note again: I’ve registered, so i guess there won’t be any captcha anymore ;)

  8. Perishable 2007/05/23 1:01 pm


    My tests indicate that Firefox 2 is indeed caching files compressed via gzip, regardless of method. Using the first method with an offset value of 3600*24*7, Firebug provides the following data when the compressed file is loaded into the browser:

    Date: Wed, 23 May 2007 19:38:05 GMT
    Cache-Control: must-revalidate
    Content-Encoding: gzip
    Expires: Wed, 30 May 2007 19:38:05 GMT

    Notice the time difference between Date and Expires – precisely one week, as specified by the offset.

    To verify further, clear your Fx2 cache and open about:cache via the address bar. After verifying a zero cache sum, load the gzipped page in question and take another look at the cache contents. The compressed file now should appear in the "Disk cache device" cache.

  9. You are totally right, and I could’ve found it out myself :/

    But anyway, it’s strange that the style.php appears in dark grey in FireBug, maybe it colors the elements just by their extensions…

    Thank you very much for your work !

  10. My pleasure — glad to be of help!

  11. August Klotz 2007/05/30 12:10 am

    Just a note:
    In the first JavaScript-compression method you employ the function ob_start("ob_gzhandler"); without first testing for the presence of the required Apache extension, zlib. Replacing that first line with a simple check is an easy way to prevent unnecessary errors during implementation. Something such as the following would definitely do the trick:

    <?php if(extension_loaded('zlib')){ob_start('ob_gzhandler');} ?>

  12. Hello Mate,
    After adopting method 1, I can see php file of the js file in my browser cache. The filesize of it remains the same. How would I understand the file has been zipped at some point? Thank you.

    Kind Regards

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Perishable Press is operated by Jeff Starr, a professional web developer and book author with two decades of experience. Here you will find posts about web development, WordPress, security, and more »
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