MongoDB empty string value vs null value

Question

In MongoDB production, if a value of a key is empty or not provided (optional), should I use empty string value or should I use null for value.

1) Is there any pros vs c

Answer 1

I think the best way is undefined as I would suggest not including this key altogether. Mongo doesn't work as SQL, where you have to have at least null in every column. If you don't have value, simply don't include the key. Then if you make query for all documents, where this key doesn't exists it will work correctly, otherwise not. Also if you don't use the key you save a little bit of disk space. Do this is the correct way in Mongo.

function deleteEmpty (v) {
   if(v==null){
     return undefined;
   }
   return v;
}

var UserSchema = new Schema({
email: { type: String, set: deleteEmpty } 
});