Check if innerHTML is empty

I have a ajax call on my onkeyup event, that checks if the given email address is in the database or not. My problem is, that I can’t find a right solution to check if the ajax response is EMPTY and if yes I need to disable a button.

This is the ajax.php output:

$sql = "// the query";

$result = mysql_query($sql);
$num_rows = mysql_num_rows($result);

if($num_rows>0){
    echo "<i>Yepp, this is taken</i>";
  }

This is the JavaScript for the ajax call AND the button disable and enable function (ajax call only if the email address is valid):

function checkEmail(str){

    email = document.getElementById('user-email').value
    AtPos = email.indexOf("@")
    StopPos = email.lastIndexOf(".")

    if (AtPos == -1 || StopPos == -1) {
         document.getElementById('button-reg').disabled=true;    
    }else{
        if (str=="")
          {
          document.getElementById("txtHint").innerHTML="";
          return;
          }
        if (window.XMLHttpRequest)
          {// code for IE7+, Firefox, Chrome, Opera, Safari
          xmlhttp=new XMLHttpRequest();
          }
        else
          {// code for IE6, IE5
          xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
          }
        xmlhttp.onreadystatechange=function()
          {
          if (xmlhttp.readyState==4 && xmlhttp.status == 200){
                document.getElementById("txtHint").innerHTML=xmlhttp.responseText;
            }
          }
        xmlhttp.open("GET","ajax.php?q="+str,true);
        xmlhttp.send();
    }
    var htmlstring = document.getElementById("txtHint").innerHTML;

    htmlstring = htmlstring.replace(/^s+/,'');
    
    if(htmlstring==""){
        document.getElementById('button-reg').disabled=false;
    }else{
        document.getElementById('button-reg').disabled=true;
    }
}

As you can see I tried with whitespace elimination, but no luck… after empty response the button is still not disabled…

Can someone help me out here?

Answer

Use this function..

function checkEmail(str){

    email = document.getElementById('user-email').value
    AtPos = email.indexOf("@")
    StopPos = email.lastIndexOf(".")

    if (AtPos == -1 || StopPos == -1) {
         document.getElementById('button-reg').disabled=true;    
    }else{
        if (str=="")
          {
          document.getElementById("txtHint").innerHTML="";
          return;
          }
        if (window.XMLHttpRequest)
          {// code for IE7+, Firefox, Chrome, Opera, Safari
          xmlhttp=new XMLHttpRequest();
          }
        else
          {// code for IE6, IE5
          xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
          }
        xmlhttp.onreadystatechange=function()
          {
          if (xmlhttp.readyState==4 && xmlhttp.status == 200){
                document.getElementById("txtHint").innerHTML=xmlhttp.responseText;

                htmlstring = xmlhttp.responseText.replace(/^s+/,'');
                if(htmlstring==""){
                    document.getElementById('button-reg').disabled=false;
                }else{
                    document.getElementById('button-reg').disabled=true;
                }
            }
          }
        xmlhttp.open("GET","ajax.php?q="+str,true);
        xmlhttp.send();
    }
}

Hope this helps..