There is a non-existent variable referenced somewhere. This variable needs to be declared, or you need to make sure it is available in your current script or scope otherwise , it will endup throwing this 'login_focus' is not defined error . This usually indicates that your library is not loaded and JavaScript does not recognize the 'login_focus'.
To solve this error: Load your library at the beginning of all your scripts.
There can be multiple other reasons for this issue:
Added JavaScript library after the script were you see that error
Conflict with Other JavaScript Libraries
Path to JavaScript library you included is not correct
The jQuery library file is corrupted
May be a chance where you are working offline