// a shortcut for html element (dom = document object model)
function dom(id) {
    return document.getElementById(id);
}

// when insert button is clicked, try to send data to server
function sendData() {
    let errorMessages = getErrorMessages();
    if (errorMessages !== "") {
        // if there are errors, show them in msg div
        dom("msg").innerHTML = errorMessages;
    } else {
        // if all values are ok
        // create an object with values
        let txtStudentName = dom("txtStudentName").value;
        let txtStudentAge = dom("txtStudentAge").value;
        let student = {
            txtStudentName: txtStudentName,
            txtStudentAge: txtStudentAge
        };
        // the xhr object will serve for performing HTTP request
        let xhr = new XMLHttpRequest();
        // the request will be of type POST and will address "/student" route
        xhr.open("POST", "/student");
        // the request will send json, UTF8 data
        xhr.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
        // transmit the object converted to JSON
        xhr.send(JSON.stringify(student));
        // every time when a change in HTTP request status occures
        xhr.onreadystatechange = function () {
            // if HTTP request is DONE
            if (xhr.readyState === 4) {
                // if status is ok
                if (xhr.status === 200) {
                    dom("msg").innerHTML = "Datele au fost salvate cu succes.";
                // if status is not ok
                // display error message and the response from server
                } else {
                    dom("msg").innerHTML = "Eroare server la salvarea datelor." + xhr.responseText;
                }
            }
        };
    }
}

function getErrorMessages() {
    // this variable will sum up all the error messages
    let errorMessages = "";
    // perform 'name' validation
    if (!isNameValid()) {
        errorMessages += "Numele trebuie să aibă lungimea cuprinsă între 3 și 50 caractere.";
    }
    // perform 'grade' validation
    if (!isAgeValid()) {
        errorMessages += "Varsta invalida.";
    }
    return errorMessages;
}

function isNameValid() {
    let txtStudentName = dom("txtStudentName");
    if (txtStudentName.value.length < 3 || txtStudentName.value.length > 50) {
        return false;
    }
    return true;
}
function isAgeValid() {
    let txtStudentAge = dom("txtStudentAge");
    if (isNaN(txtStudentAge.value) || txtStudentAge.value < 1 || txtStudentAge.value > 100) {
        return false;
    }
    return true;
}
function showStudentPage(){
    dom("pgStudent").style.display="block";
   dom("pgStudents").style.display="none";
   dom("pgLogin").style.display="none";
}
function showStudentsPage(){
    dom("pgStudent").style.display="none";
   dom("pgStudents").style.display="block";
   dom("pgLogin").style.display="none";
}
function showLoginPage(){
    dom("pgStudent").style.display="none";
   dom("pgStudents").style.display="none";
   dom("pgLogin").style.display="block";

}
function hideMessagePage(){
    dom("pgMessage").style.display="none";
}


??????????????????????
let express = require('express');
let path = require('path');
let bodyParser = require('body-parser');
let mysql = require('mysql');

let app = express();

app.use(express.static(path.join(__dirname, 'public')));

app.use(bodyParser.json());
// define rutes
app.post('/student', function (req, res) {
    // get data from client request
    let name = req.body.txtStudentName;
    let age = req.body.txtStudentAge;

const CONNECTION_PARAMS = {
    host: "localhost",
    port: 55555,
    user: "root",
    password: "sa",
    database: "firstdb"
};

let con = mysql.createConnection(CONNECTION_PARAMS);
    // open the connection
    con.connect(function (err) {
        // if connection error occurs, the client must be informed
        if (err) {
            res.status(500).send("Eroare la conectarea la baza de date." + err.message);
        } else {
            // prepare the insert query
            let sql = "INSERT INTO second (name, age) VALUES ('" + name + "','" + age + "');";
            // run query
            con.query(sql, function (err, result) {
                // if query error occurs, the client must be informed
                if (err) {
                    res.status(500).send("Eroare la inserarea datelor." + err.message);
                // if we are here => Hooray, no errors!
                } else {
                    res.status(200).send("");
                }
            });
        }
    });
});

app.get('/student', function (req, res) {
    // get data from client request
    let txtStudentName = req.body.txtStudentName;
    let txtStudentAge = req.body.txtStudentAge;

const CONNECTION_PARAMS = {
    host: "localhost",
    port: 55555,
    user: "root",
    password: "sa",
    database: "firstdb"
};

let con = mysql.createConnection(CONNECTION_PARAMS);
    // open the connection
    con.connect(function (err) {
        // if connection error occurs, the client must be informed
        if (err) {
            res.status(500).send("Eroare la conectarea la baza de date." + err.message);
        } else {
            // prepare the insert query
            let sql = "SELECT * FROM students";
            // run query
            con.query(sql, function (err, result) {
                // if query error occurs, the client must be informed
                if (err) {
                    res.status(500).send("Eroare la inserarea datelor." + err.message);
                // if we are here => Hooray, no errors!
                } else {
                    res.status(200).send(result);
                }
            });
        }
    });
});

// Define the static folder
app.use(express.static(path.join(__dirname, 'public')));

// Inform that server is alive
console.log("Server running on 8080.");
// Listen to a port
app.listen(8080);