Custom Live Validation Function

I've been trying to create my own custom LiveValidation (LiveValidation.com) function that connects to a database and checks if a username already exists. This is the relevant part of the form:

    Username: <input type="text" name="username" id="username" class="textinput"> 
    <script type="text/javascript"> 
        var username = new LiveValidation('username');
        username.add( Validate.Presence );
        username.add( Validate.Length, { minimum: 3, maximum: 12 } );
  开发者_如何学编程      username.add( Validate.Username );  
    </script>

This is my Validate.Username function:

Validate.Username = function(value, paramsObj){

        var paramsObj = paramsObj || {};
        var message = paramsObj.failureMessage || "Sorry that username is taken!";

    var http = new XMLHttpRequest();
    var url = "usernamecheck.php";
    var params = "username="+value;
    http.open("POST", url, true);
    http.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    http.setRequestHeader("Content-length", params.length);
    http.setRequestHeader("Connection", "close");
    http.onreadystatechange = function() {
        if(http.readyState == 4 && http.status == 200) {
            if(http.responseText == 'yes') {
                return true;
            } else {
                Validate.fail(message);
                return false;
            }
        }
    }
    http.send(params);
}

Here's the relevant part of usernamecheck.php:

if(mysql_num_rows($query) != 0 && $query) {
            echo "no";
        } else if ($query) {
            echo "yes";
        } else {
            echo "err";
        }

The function seems to work fine (if I put alert('fail') above Validate.fail(message); I get an alert when a used username is used), but LiveValidation never shows the error message. Anyone know what I've done wrong?

Thanks

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Hmmm... just looking at it real quick, could it be that "message" is not defined in your function ? It is defined in the parent function.