This only creates a new window if there is already one, not if none exists.
The following, which seems like the correct logic, does nothing. Thanks for any help. (Seems like this should be an OS option, but I don't see it. El Capitan)
I reckon too that “Home Directory” is a more comfortable idiom for most folks to use when they're fooling with an alien contraption like a computer.
var fi = Application('Finder'),
ws = fi.windows;
if (ws.length) ws[0].activate();
else {
var a = Application.currentApplication(),
sa = (a.includeStandardAdditions = true, a);
fi.open(
sa.pathTo('documents folder')
);
}