// holds an instance of XMLHttpRequest
var xmlHttp = createXmlHttpRequestObject();
 
// creates an XMLHttpRequest instance
function createXmlHttpRequestObject()
{
  // will store the reference to the XMLHttpRequest object
  var xmlHttp;
  // this should work for all browsers except IE6 and older
  try
  {
    // try to create XMLHttpRequest object
    xmlHttp = new XMLHttpRequest();
  }
  catch(e)
  {
    // assume IE6 or older
    var XmlHttpVersions = new Array("MSXML2.XMLHTTP.6.0",
                                    "MSXML2.XMLHTTP.5.0",
                                    "MSXML2.XMLHTTP.4.0",
                                    "MSXML2.XMLHTTP.3.0",
                                    "MSXML2.XMLHTTP",
                                    "Microsoft.XMLHTTP");
    // try every prog id until one works
    for (var i=0; i<XmlHttpVersions.length && !xmlHttp; i++)
    {
      try
      {
        // try to create XMLHttpRequest object
        xmlHttp = new ActiveXObject(XmlHttpVersions[i]);
      }
      catch (e) {}
    }
  }
  // return the created object or display an error message
  if (!xmlHttp)
    alert("Error creating the XMLHttpRequest object.");
  else
    return xmlHttp;
}

// called to read a file from the server
function GetData(divID,RType,KeyData)
{
  // only continue if xmlHttp isn't void <div>
  if (xmlHttp)
  {
    // try to connect to the server
    try
    {
      // initiate reading the async.txt file from the server
      // 在這邊指定呼叫 GetData.php，並使用 GET 的方式處理 Server 端的資料
      var strURL = 'GetData.php?RType=' + RType + '&KeyData=' + KeyData;
      strURL += "&parm=" + new Date().getTime();
      xmlHttp.open("GET", strURL, true);
      xmlHttp.onreadystatechange = function()
      {
        // obtain a reference to the <div> element on the page
        // 取得指定 id 的 div 物件
        myDiv = document.getElementById(divID);
        if (xmlHttp.readyState == 4)
        {
          // 利用 HTTP 狀態碼，判斷 Server 端回傳資料的狀態
          if (xmlHttp.status == 200)
          {
            try
            {
              // read the message from the server
              response = xmlHttp.responseText;
              // display the message
              // 將回傳的資料，寫入 div 物件中
              myDiv.innerHTML = response;
            }
            catch(e)
            {
              // display error message
              alert("Error reading the response: " + e.toString());
            }
          }
          else
          {
            // display status message
            alert("There was a problem retrieving the data:n" + xmlHttp.statusText);
          }
        }
      }
      xmlHttp.send(null);
    }
    // display the error in case of failure
    catch (e)
    {
      alert("Can't connect to server:n" + e.toString());
    }
  }
}