Check if _ppvPreviousPage !== 'undefined'

In my understanding you need to check if _ppvPreviousPage !== 'undefined' when you "Install the plug-in using tags in Adobe Experience Platform" as in this case _ppvPreviousPage returns 'undefined' when not set.

if(_ppvPreviousPage)
if(_ppvPreviousPage && (_ppvPreviousPage !== 'undefined'))
{
// DO WHATEVER YOU LIKE
}

Start by locating the documentation section titled "Install the plug-in using tags in Adobe Experience Platform" and review how _ppvPreviousPage is described. Done means the guidance clearly covers the 'undefined' value and shows the intended condition consistently.

Written by the indexing model from the issue text.