Q:

gzip js on apache

the following configuration in httpd.conf only gzip css and html, not javascript, any idea? 
AddOutputFilterByType DEFLATE text/html text/plain text/javascript text/css
AddOutputFilterByType DEFLATE application/x-javascript

A:

That's most likely because the MIME type for Javascript is hotly contested. 
Try adding application/javascript and text/x-js (as well as application/x-javascript)
That said, a more reliable way may be to filter based on the .js file extension.  See the Request_URI directive for more info (http://httpd.apache.org/docs/2.0/mod/mod_deflate.html) - this may not work if you're using rewritten URLs that do not use the .js extension for JavaScript files. 
If this doesn't work, then your distro of Linux may be using a completely different MIME type for JavaScript.  Locate the TypesConfig directive in your config file, and have a look to see how it's defining JavaScript.  Then add that MIME type in as well.