Error : UTF8 is not a function ,after adding a username and password to mongodb connection string

I have created one sample application for connection of mongodb and nodejs. I got error like MongoError: there are no users authenticated

after that I have added username and password to the mongodb connection string like "mongodb://root:root@localhost:27017/marankings"

so authentication eror has gone and it gives me new error like

Buffer.from('n=', 'utf8'), ^

TypeError: utf8 is not a function at Function.from (native) at Function.from (native)

Please guide me on this , I am stuck here.

This is my code

connectToServer: function(callback) {
  MongoClient.connect("mongodb://root:root@localhost:27017/marankings", function(err, db) {
    //db = db;
    var dbcon=db.db("marankings");
    console.log("this is connected db"+_db);
    var myobj = { name: "Akshay Champavat", address: "[email protected]" };

    dbcon.collection("users").insertOne(myobj, function(err, res) {
      if (err) throw err;
      console.log("1 document inserted");
      dbcon.close();
    });

    return callback(err);
  });
},

Please help , Thanks in advance !!!