Javascript Alert showing blank message

function getXMLHTTPRequest()
{
var req = false;
try
  {
   req = new XMLHttpRequest(); /* e.g. Firefox */
  }
catch(err1)
  {
  try
    {
     req = new ActiveXObject("Msxml2.XMLHTTP");  /* some versions IE */
    }
  catch(err2)
    {
    try
      {
       req = new ActiveXObject("Microsoft.XMLHTTP");  /* some versions IE */
      }
      catch(err3)
        {
         req = false;
        }
    }
  }
return req;
}
 
 
 
 
 
 
 
function issubmissionvalid(str)
// Ajax code which posts values to getrefno.php
{
   xmlHttp=getXMLHTTPRequest()
   if (xmlHttp==null)
   {
       alert ("Browser does not support HTTP Request");
       return;
   }
  var dbaction = str;
  var refnosubmitted = document.forms['newrefdetails'].elements['refno'].value;
  var apptypesubmitted = document.forms['newrefdetails'].elements['apptype'].value;
  var refstartsubmitted = document.forms['newrefdetails'].elements['refstart'].value;
  var refendsubmitted = document.forms['newrefdetails'].elements['refend'].value;
  //alert ("the values submitted are: a=" + apptypesubmitted + " b=" + refstartsubmitted + " c=" + refendsubmitted);
  url='getrefno.php'+"?a="+ apptypesubmitted + "&b=" + refstartsubmitted + "&c=" + refendsubmitted +"&d=" + refnosubmitted + "&e=" + dbaction;
  url=url+"&sid="+Math.random();
  xmlHttp.onreadystatechange=stateChangedref;
  xmlHttp.open("GET",url,true) ;
  xmlHttp.send(null);
   
}
 
 
function stateChangedref()
{
   if (xmlHttp.readyState==4 )
   {
           var mystring = xmlHttp.responseText;
           alert (mystring);
   }
   else { }
}
 
 
 
 
 
 
function confirmupdate(str)
//this function acts as a confirmation request to check if the user really wants to update the reference number
{
var errornum = 0;
 
if (IsNumeric(document.forms['newrefdetails'].elements['refstart'].value) == false)
{errornum = (errornum + 1);}
if (IsNumeric(document.forms['newrefdetails'].elements['refend'].value) == false)
{errornum = (errornum + 1);}
if (errornum > 0)
{alert ("Check that the values you entered are valid numbers and try again");
}
 
else
{
input_box=confirm("ARE YOU SURE YOU WISH TO CONTINUE?  .... Click 'OK' to update the record or click 'Cancel' to cancel the update");
    if (input_box==true)
    {
    issubmissionvalid(str);
        window.opener.location.reload();
        window.close();
    }
 
    else
    {
    // Output when Cancel is clicked
    alert ("The update has been cancelled");
    return false;
    }
    }
}
 
 
function IsNumeric(strString)
   //  check for valid numeric strings
   {
   var strValidChars = "0123456789";
   var strChar;
   var blnResult = true;
   if (strString.length == 0) return false;
   //  test strString consists of valid characters listed above
   for (i = 0; i < strString.length && blnResult == true; i++)
      {
      strChar = strString.charAt(i);
      if (strValidChars.indexOf(strChar) == -1)
         {
         blnResult = false;
         }
      }
   return blnResult;
   }

                                  

1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
24:
25:
26:
27:
28:
29:
30:
31:
32:
33:
34:
35:
36:
37:
38:
39:
40:
41:
42:
43:
44:
45:
46:
47:
48:
49:
50:
51:
52:
53:
54:
55:
56:
57:
58:
59:
60:
61:
62:
63:
64:
65:
66:
67:
68:
69:
70:
71:
72:
73:
74:
75:
76:
77:
78:
79:
80:
81:
82:
83:
84:
85:
86:
87:
88:
89:
90:
91:
92:
93:
94:
95:
96:
97:
98:
99:
100:
101:
102:
103:
104:
105:
106:
107:
108:
109:
110:
111:
112:
113:
114:
115:
116:
117:
118:
119:
120:
121:
122:
123:
124: