Express.js handler to update user fields
Assignments can be contracted to something like below:

 req.user.firstName = req.body.firstName || req.user.firstName;
 req.user.lastName = req.body.lastName || req.user.lastName;
 req.user.email = req.body.email || req.user.email;
 req.user.phone = req.body.phone || req.user.phone;
 req.user.fax = req.body.fax || req.user.fax;
 req.user.notes = req.body.notes || req.user.notes;


I think it can further be reduced if properties are put in some array and iterated over (since source and destination property have the same name), but I would stick with this arguably more readable text.

I am not familiar with express.js, so the following might not be applicable (I usually work with Typescript in Angular).

Your request have both a body and some user property. It is recommended to clearly separate output from input in order to obtain pure functions as much as possible. Something like below:

 updateUserFromRequest(req, user) {

     // clone to avoid changing provided reference
     var ret = [... user];

     // assignment logic here using ret

     return ret;
 }