Remember that you are only testing/previewing your user JavaScript. It has not yet been saved!
The Mer Wiki now uses your Mer user account and password (create account on https://bugs.merproject.org/)
MediaWiki:Userjspreview
From Mer Wiki
The Mer Wiki now uses your Mer user account and password (create account on https://bugs.merproject.org/)
Remember that you are only testing/previewing your user JavaScript. It has not yet been saved!