One of my clients reported my an issue today that switching tabs in their editor stopped working, we’re talking about the “Visual” and “Text” tabs here.When clicking on one of the tabs, the following JavaScript error appeared in my console:

Uncaught TypeError: Cannot call method 'switchto' of undefined

For those running into the same issue, adding the following line to your wp-config.php file will solve your issue.

define( 'CONCATENATE_SCRIPTS', false );

Please note while this might solve this issue, the real problem probably lies somewhere else. Most likely there is a plugin ‘misbehaving’. You can simply find out if this is the case by disabling all plugins and see if this fixes the problem. Don’t forget to remove the define mentioned above if you’re going on this debugging route 😉