AJAX xmlhttp.responseText

Question

I've been racking my brain for a few hours and can't figure out why a string comparison won't work. In the following code, I do a xmlhttp call and get a response text. The PHP file that I get a call IS returning the proper response string "NOAD", and NOAD is being displayed when appropriate in my testing. However, when the call is returned NOAD I want to identify it, however for some reason within the call below xmlhttp.responseText == comparisonText its NOT properly comparing the two. Why does xmlhttp.responseText printout NOAD but I can't use it within the comparator?

function loadXMLAdImage1Doc(currentScenarioTime)
{

var returnText = "Not Here";
var comparisonText = "NOAD";

var xmlhttp;
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)
{


    if (xmlhttp.responseText == comparisonText) 
       {
            document.getElementById("AJAXTEST").innerHTML =returnText;
       } else  {

            document.getElementById("AJAXTEST").innerHTML =xmlhttp.responseText; 
            }
    }
  }

Answer 1

Okay, thanks Luth for pointing me in the right direction. I solved my problem, but I'm not sure what the problem is within xmlhttp.responseText or user error that caused it.

The return text from my PHP file was placing an unseen character BEFORE the returned string (in testing, I would have figured it was an invisible character after prior to the results but go figure). Theoretically, this shouldn't be happening...my PHP is returning a straightforward string:

..do some MYSQL queries..
$AdLink = mysql_result($result, 0);

if ($AdLink == "") {

echo "NOAD";
} else
{
echo $AdLink;
}

So the PHP file SHOULD be sending a 4 character return screen to the xmlhttp.responseText call named "NOAD". But what was found in measuring the string length was that it was sending a 5 character string back with some invisible character BEFORE "NOAD" that wasn't showing up on the screen - therefore screwing up the comparator. I used the following code to remove the character and it worked perfectly...

returnText = xmlhttp.responseText;
var returnlength = returnText.length;

returnText = returnText.substring(1, returnlength);

I have no idea WHAT the character is, whether its a problem with my PHP code or the xmlhttp.responseText call, but something to be aware of if you're dealing with the call.