用javascript实现AJAX功能(不需要xmlhttp对象)

function AjaxScript(url, varName, params, onsuccess)
{ 
 var arr = document.getElementsByTagName("script");
 var url =url.toLowerCase();
 var isLoad = true;
 if (arr)
 {
  for(i=0; i<arr.length; i++)
  {
   var src = arr[i].src;
   if (src)
   {
    if (src.toLowerCase() == url) { isLoad = false; }
   }
  }
 }
 
 if (isLoad)
 {
  var s = document.createElement("script");
  var params = (params ? params : "") + (params && varName ? "&" : "") + (varName ? "varName=" + varName : "")
  if (url.indexOf("?") >= 0)
  {
   url += (params && params != "" ? "&" + params  : "");
  }
  else
  {
   url += (params && params != "" ? "?" + params : "");
  }
  
  s.src = url;
  s.type = "text/javascript";
  s.onreadystatechange = function()
  { 
   switch(this.readyState)
   {
    case "complete":
    case "loaded":
     var cmd = "try { if (onsuccess) { onsuccess({0});  {1} } }catch(e){}".format((varName != null ?  varName : ""), (varName != null ? "delete {0}".format(varName) : "") )
     eval(cmd);
     break;
     
    case "loading":
     break;
   }
  }
  s.onload = function() {
     var cmd = "try { if (onsuccess) { onsuccess({0});  {1} } }catch(e){}".format((varName != null ?  varName : ""), (varName != null ? "delete {0}".format(varName) : "") )
     eval(cmd);
    }
  var arr = document.getElementsByTagName("head");
  var h= arr[0];
  document.body.appendChild(s);
 }
 else
 {
  eval("if (onsuccess) { onsuccess({0}); }".format( varName != null ?  varName : "" ));
 }
}