“Object doesn’t support this property or method” errors in jQuery UI

Ever had an “Object doesn’t support this property or method” javascript error occuring inside your downloaded custom jQuery UI code file, e.g.: jquery-ui-1.8.5.custom.min.js?

If you’ve confirmed that jQuery itself is working fine, you might be missing an internal dependency. Apparently, sometimes the jQuery UI download page’s dependency checker can get it wrong (in my case, draggable was listed as a dependency in the comments in the code for dialog, but it wasn’t enforced as such when downloading.).

Try going back and downloading it again, but this time check everything. Replace your existing jQuery UI custom js file with the new one, and see if that fixes the problem.