inserting data in mongoose using nodejs

I am using nodejs to insert data in mongoose db. When i tried to run the .js file using the cmd, it just goes idle and does nothing. Below is my code that i am using:

var mongoose = require('mongoose');
var db = mongoose.createConnection('localhost','userregistration');

db.once('open',function(){
    var newuserSchema = new mongoose.Schema({
       firstname: String,
        Lastname: String,
        username: String,
        Email: String,
        Password: String
    });

    var newuser = db.model('newuser',newuserSchema);
    var newuseradd = new newuser({
        firstname: 'aa',
        Lastname: 'aa',
        username: 'aa',
        Email: 'aa@yahoo.com',
        Password: 'as'
    });

   // console.log(newuser);
    newuseradd.save();

});