how to check uniqueness of a username in MySQL database via JSON? -

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i have next page, on user can create new account. question how can make sure user inserts username doesn't exist in mysql database. page available on http://webs.hogent.be/kevinbaeyens/gebruiker.html code of page is:

<!doctype html>  <html class="">  <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1"> <title>nieuwe gebruiker | sociale buurt</title> <link href="boilerplate.css" rel="stylesheet" type="text/css"> <link href="onzebuurt.css" rel="stylesheet" type="text/css">  <script type="text/javascript" src="jquery-1.7.1.min.js"></script> <script type="text/javascript" language="javascript">     window.onload = function() {       document.getelementbyid("username").focus();     };      function formuliervalideren() {                  if (document.getelementbyid('username').value == '' || document.getelementbyid('username').value == null)             {                 alert ('gebruikersnaam verplicht.');                 document.getelementbyid('username').style.bordercolor = "red";                 return false;             }             else if (document.getelementbyid('wachtwoord').value == '' || document.getelementbyid('wachtwoord').value == null)             {                 alert ('wachtwoord verplicht.');                 document.getelementbyid('wachtwoord').style.bordercolor = "red";                 return false;             }             else if (document.getelementbyid('wachtwoord2').value == '' || document.getelementbyid('wachtwoord2').value == null)             {                 alert ('bevestig wachtwoord.');                 document.getelementbyid('wachtwoord2').style.bordercolor = "red";                 return false;             }             else if (document.getelementbyid('wachtwoord2').value != document.getelementbyid('wachtwoord').value)             {                 alert ('wachtwoorden komen niet overeen.');                 document.getelementbyid('wachtwoord2').style.bordercolor = "red";                 return false;             }             else             {                 var url = "http://localhost:8080/onzebuurt2/resources/gebruikers";                  var reminder = {};                 reminder.naam = jquery.trim($("#username").val());                 reminder.wachtwoord = jquery.trim($("#wachtwoord").val());                  var request = new xmlhttprequest();                 request.open("post", url);                 request.onload = function() {                     if (request.status === 201) {                         reminder.id = request.getresponseheader("location").split("/").pop();                       } else {                         console.log("error creating reminder: " + request.status + " " + request.responsetext);                     }                 };                  request.setrequestheader("content-type", "application/json");                 request.send(json.stringify(reminder));                          var msg = "registratie succesvol. klik op ok om u aan te melden op de site.";                 if(confirm(msg)){                 settimeout(function() {window.location.href = "http://localhost:8080/onzebuurt2/"})                 }              }             //end if     }//end function   </script> </head> <body class="body2"> <div class="gridcontainer clearfix">   <div class="header2">         <center>             nieuwe gebruiker         </center>   </div>         <div id="formulier2">             <form method="post" name="form" action="">                 <p class="labels"><center>gebruikersnaam *</center></p><input id="username" type="text" name="username" placeholder="gebruikersnaam" size="50">                 <p class="labels"><center>wachtwoord *</center></p><input id="wachtwoord" type="password" name="wachtwoord" placeholder="wachtwoord" size="50">                 <p class="labels"><center>bevestig wachtwoord *</center></p><input id="wachtwoord2" type="password" name="bevestig wachtwoord" placeholder="bevestig wachtwoord" size="50">                 <br />                 <a href="index.html" style="text-decoration:none"><center><img id="return" name="jsbutton" src="return.png" alt="terug" /></center></a>                 <br />                 <center><input id="bevestig" type="image" src="bevestig.png" width="200"  height="50" border="0" alt="submit!" onclick="formuliervalideren()"></center>                 <br />             </form>         </div> </div> </body> </html>

some notes need know:

1) since it's assignment school, can't use php.

2) primary key in database table 'gebruiker' 'id', field of 'username' field has unique

thanks in advance!


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