validationLogin.js

function validateLogin() {
    var res = true;
    res = username() && res;
    res = password() && res;
    return res;
}

function username() {
    const username = document.getElementById("Username").value;
    const usernameError = document.getElementById("Username_ErrorMSG");

    if (username == "") {
        usernameError.innerHTML = "Enter a Username!";
        return false;
    }

    return true;
}

function password() {
    const password = document.getElementById("Password").value;
    const passwordError = document.getElementById("Password_ErrorMSG");

    if (password == "") {
        passwordError.innerHTML = "Enter a Password!";
        return false;
    }

    return true;
}