I got a function in javascript that first checks if the username is the required length and than sends the username to a page for a test if the test is a success there will be a <span id="username">true</span> else <span id="username">false</span>

Ben getElementById dönüş veri yoksa bir hata almaya devam

  function checkuser(user, wd,id, sub)
    {
        if(user.length < 7)
        {
            document.getElementById(id).innerHTML = "Your username is too short. It must be longer than 6 charaters";
            document.getElementById(sub).disabled = true;
            return false;
        } else {
            $.post(wd + "register/checkuser/" + user,
                       function(data){
                         alert("Data Loaded: " + data.getElementById('username').innerHTML;
                       });
        }
    }