Script loader updates

There are several updates to the script loader currently in WordPress 2.8-bleeding-edge that enhance and optimize loading of external JavaScript and CSS files.

Probably the most important change is that scripts can be queued for loading in the footer for both the admin and the front-end. This is done with an optional argument. To enqueue a script for the footer:

wp_enqueue_script( 'name', 'url/to/file.js', array('dependency'), 'version', true );

where “true” means enqueue for the footer (“false” is the default and is optional).

When a script is enqueued for the footer all dependencies will be added (if not already present) and will be printed before the script. Some may be in the head, others also in the footer. By default only jQuery is printed in the head but when a script is enqueued for the head, all dependencies would also be printed in the head. Almost all external scripts would run onload or after the page has loaded, so there’s no real need to queue anything for the head.

Scripts queued for the front-end footer depend on wp_footer(); being present in the current theme. Unfortunately some themes don’t include it. The best way to remedy this would be to bring awareness among users and theme designers as suggested by several plugin developers.

To make queueing of scripts easier two new actions have been added: "wp_enqueue_scripts" that runs in the front-end head where all is_page(), is_home(), etc. functions are available and "admin_enqueue_scripts" that runs in the admin head and has the current page hook as argument, so scripts can be queued only for specific pages.

Another major new feature is that all core admin scripts are concatenated and compressed before sending them to the browser. This feature can easily be extended to include scripts added by plugins and to use server side caching, however that would require some changes to the server settings (.htaccess on Apache).

Since compression from php can be problematic on some hosts there are several “switches” (constants) that manage it: define('CONCATENATE_SCRIPTS', false); would turn off both concatenating and compressing of all scripts. It’s intended for script debugging, define('COMPRESS_SCRIPTS', false); can be used to turn off compression for JavaScript and define('COMPRESS_CSS', false); for CSS files. Compression is set to “deflate” by default since it’s faster and uses a little less server resources. Gzip can be forced by setting define('ENFORCE_GZIP', true);

There is a test if compressing from php works as expected on the server and whether the server compresses scripts by default. It runs only once and saves the result in an option “can_compress_scripts”. It would run again if the option is deleted.

In addition all core scripts are minified. All custom scripts are included in two versions: .dev.js is the non-minified script and .js is the minified one. The constant define('SCRIPT_DEBUG', true); would load the .dev.js versions of the scripts making them easier to debug.

Possible changes: removing the COMPRESS_CSS switch and using only COMPRESS_SCRIPTS, using deflate for compression but adding the gzip file header and serving it as “Content-Encoding gzip” since it seems more compatible with the various web servers and proxyes (all modern browsers support deflate well).