Ajax on ASP

Hi experts, I wish to put some alert where if the client cannot able to connect the data server, there will be a message to allow the client know that the server is busy or Server does not exist or access denied.. I am using mssql 7. I have no idea how hard or possible is this. any help please!

function showCustomer(str) {
  var xhttp;
  if (str == "") {
    document.getElementById("txtHint").innerHTML = "";
    return;
  }
  xhttp = new XMLHttpRequest();
  xhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
    document.getElementById("txtHint").innerHTML = this.responseText;
    }
  };
  xhttp.open("GET", "getcustomer.asp?q="+str, true);
  xhttp.send();
}

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