js写ajax并解析json

function down(t){
 
 var req = createRequest();  //创建request
 req.open("GET","selectWordDate?date_time="+t);  //发送请求
 req.onreadystatechange = function(){
  callback(req);
 }
 req.send(null);
}

function createRequest()
{
 if(window.XMLHttpRequest)//ns
 {
  return new XMLHttpRequest();
 }else//IE
 {
  try{
      return new ActiveXObject("Msxml2.XMLHTTP");
  }catch(e){
   return new ActiveXObject("Microsoft.XMLHTTP");
  }
 }
 return null;
}

function callback(req){

if(req.readyState == 4) {   if(req.status != 200){    return;   }

var json = req.responseText;

var o = JSON.parse(json);

var datas=o.obj;

var fileName = datas.wordName;

var filePath = datas.fileUrl;

if(filePath!=""){

window.location = "downloadFile?fileName=" + fileName + "&filePath=" + filePath;

}

else{

alert("文件不存在");    return false;

}

}

}