User data storage

Question

I am writing my own website where users sign up. It then saves that data in an array. I want to know if this is effective or if there are better ways to do it.

var isLoggedOn = false;
var numberOfUsers = 0;
var users = new Array();
var correct = false;
var currentUser = new User();
var date = new Array();
var userNumber = 0;

function getRegistrationFields() {
    var first1 = document.getElementById("firstname");
    first1.onclick = getRegistrationFields;
    var last1 = document.getElementById("lastname");
    last1.onclick = getRegistrationFields;
    var email1 = document.getElementById("email");
    email1.onclick = getRegistrationFields;
    var age1 = document.getElementById("age").value;
    age1.onclick = getRegistrationFields;
    var username1 = document.getElementById("username");
    username1.onclick = getRegistrationFields;
    var password1 = document.getElementById("password");
    password1.onclick = getRegistrationFields;
    var confirm1 = document.getElementById("passwordconfirm");
    confirm1.onclick = getRegistrationFields;
    var first = document.getElementById("firstname").value;
    var last = document.getElementById("lastname").value;
    var email = document.getElementById("email").value;
    var age = document.getElementById("age").value;
    var username = document.getElementById("username").value;
    var password = document.getElementById("password").value;
    var confirmpassword = document.getElementById("passwordconfirm").value;
    var empty = "";

    if ((first === empty) || (last === empty) || (email === empty) || (age === empty)
        || (username === empty) || (password === empty) || (confirm === empty)) {
        var message = "Not all of the fields are filled out";
        alert(message);
        //showModalWindow("fields");
        return false;
    }
    else {
       age = parseInt(age);

       if (password.length < 10) {
            var message2 = "Password must be at least 10 characters long";
            alert(message2);
            //showModalWindow("passwordlength");
            return false;
       } 
       else if (password != confirmpassword) {
            var message3 = "Passwords Do not match";
            alert(message3);
            //showModalWindow("passwordmatch");
            return false;
       }
       else if (age < 18) {
            var message4 = "You must be older to register for this software.";
            alert(message4);
            //showModalWindow("young");
            return false;
       }
       else {
            var message5 = "All of the fields are correct. We will be processing your request";
            alert(message5);
            var d = new Date();
            date[numberOfUsers] = d.toDatestring();
            var user = new User(username, password, email, first, last, age, date[numberOfUsers]);
            users[numberOfUsers]=user;
            numberOfUsers += 1;
            //showModalWindow("success");
            return true;
       }
    }
}


function User(username, password, email, firstname, lastname, age, datecreated) {
   this.username = username;
   this.password = password;
   this.firstname = firstname;
   this.lastname = lastname;
   this.name = firstname + " " + lastname;
   this.age = age;
   this.datecreated = datecreated;
}

Array.prototype.checkFields = function(username, password) {
    for(var i = 0; i < this.length; i++) {
        if(this[i].username === username) {
            if(this[i].password === password) {
                return true; 
                userNumber = i;
            }
         }
         else {
            return false;
        }
    }
}

function CheckSignIn() {
    var username = document.getElementById("username").value;
    var password = document.getElementById("password").value;

    if (username === "" || password === "") { alert("Please enter a username/password"); }
    else {

         if (users.checkFields(username, password)) { 
             window.location = "http://localhost/profile.html";
             currentUser.username = users[userNumber].username;
             currentUser.password = users[userNumber].password;
             currentUser.age = users[userNumber].age;
             currentUser.firstname = users[userNumber].firstname;
             currentUser.lastname = users[userNumber].lastname;
             currentUser.datecreated = users[userNumber].datecreated;
             isLoggedOn = true;
        }
    }
}

Answer 1

Just a few general notes:

1. Instead of using arrow if-else { if-else { ... } } structure you could format the code as guard clauses. For example:

   if ((first === empty) || (last === empty) || (email === empty) || (age === empty)
               || (username === empty) || (password === empty) || (confirm === empty)) {
       var message = "Not all of the fields are filled out";
       alert(message);
       return false;
   }
   age = parseInt(age);
   ...
   

   Reference: Flattening Arrow Code

2. You could create some local variables:

   currentUser.username = users[userNumber].username;
   currentUser.password = users[userNumber].password;
   currentUser.age = users[userNumber].age;
   ...
   

   Here users[userNumber] could be extracted out to a local variable. It would improve readability and remove some duplication.

3. Using alert does not seem good: "Alert is bad" - really?.