Mongoose only update document value if its defined

I would like to only update a value in the document if its newly defined. If its not defined, leave the value as is in the document. Example below.

If this document exists in the db

{
  "_id": "592c53b3bdf350ce004ad717",
  "url": "hyoh7ryb-",
  "fname": "Foo",
  "lname": "Bar",
  "email": "foobar@gmail.com",
  "__v": 0,
  "verfied": false,
  "bio": {
    "title": "Software Engineer",
    "intro": "I like to code stuff.",
    "summary": "Specializing in web development. Favorite language is Javascript.\n"
  }
}

My code:

module.exports.updateBio = function(req, res) {


    var userID = req.params.id;
    var objForUpdate = {bio: {}};

    if (!troolr.isEmptyString(req.body.profile_picture)) objForUpdate.bio.profile_picture = req.body.profile_picture;
    if (!troolr.isEmptyString(req.body.title))           objForUpdate.bio.title = req.body.title;
    if (!troolr.isEmptyString(req.body.intro))           objForUpdate.bio.intro = req.body.intro;
    if (!troolr.isEmptyString(req.body.summary))         objForUpdate.bio.summary = req.body.summary;   
    if (!troolr.isEmptyString(req.body.skills))          objForUpdate.bio.skills = req.body.skills;
    if (!troolr.isEmptyString(req.body.facebook))        objForUpdate.bio.social.facebook = req.body.facebook;
    if (!troolr.isEmptyString(req.body.twitter))         objForUpdate.bio.social.twitter = req.body.twitter;
    if (!troolr.isEmptyString(req.body.linkedin))        objForUpdate.bio.social.linkedin = req.body.linkedin;
    if (!troolr.isEmptyString(req.body.website))         objForUpdate.bio.social.website = req.body.website;

    var conditions = { _id: userID }
      , setObj = { $set: objForUpdate }
      , options = { multi: true };

    Profile.update(conditions, setObj, (error, page) =>{
        if(error) return res.status(500).json({ success: false, error: error });

        res.status(200).json({ success: true, message: "Bio successfully updated." });
    });
};

In my body send request, i am only updating title, but i would want to keep all the other bio info as is, but instead it creates a blank field for them.

How can I go about handling this?

via Shivam