function checkUser(userData) {
    return User.find({ userName: userData.userName }).exec().then((user) => {
        if (user == undefined || user.length == 0) {
            throw new Error("Unable to find user: " + userData.userName);
        }
        else if (user[0].password != userData.password) {
            throw new Error("Incorrect Password for user: " + userData.userName);
        }
        else if (user[0].password == userData.password)
        {
            user[0].loginHistory.push({
                dateTime: (new Date()).toString(),
                userAgent: userData.userAgent});
            user[0].update({
                userName: userData.userName}, {$set: {loginHistory: user[0].loginHistory}}).exec().then((user) => {
                return user[0];
            })
            .catch((err) => {
                console.log(err);
                console.log(err.stack);
               throw new Error("There was an error verifying the user: ${err}");
            })
        }
    }).catch((err) => {
        console.log(err);
        console.log(err.stack);
        throw new Error("Unable to find user: " + err);
    });
};

var mongoose = require("mongoose");
var Schema = mongoose.Schema;

var userSchema = new Schema({
    "userName": {
        unique: true,
        type: String
    },
    "password": String,
    "email": String,
    "loginHistory": [{ "dateTime": Date, "userAgent": String }]
});

let User; //to be defined on new connection