unable to add property to the json object

Try to .toObject() the form:

Form.findOneAndUpdate(condition, req.body, {upsert:true}, function(err, form){

    if (err) return res.send(500, { error: err });

    var objForm = form.toObject();

    objForm.status = "saved successfully";

    return res.send(objForm);

});

Mongoose query result are not extensible (object are frozen or sealed), so you can't add more properties. To avoid that, you need to create a copy of the object and manipulate it:

var objectForm = Object.create(form);
objectForm.status = 'ok';

Update: My answer is old and worked fine, but i will put the same using ES6 syntax

const objectForm = Object.create({}, form, { status: 'ok' });

Another way using spread operator:

const objectForm = { ...form, status: 'ok' }