无依赖使用Ajax获取数据的方法

1. [代码]ajax调用方法    

function ajax(url,callback){
    var xhr; 
    try {xhr = new ActiveXObject('Msxml2.XMLHTTP');   }
    catch (e){
        try {   
            xhr = new ActiveXObject('Microsoft.XMLHTTP');    
        }catch (e2){
          try {  xhr = new XMLHttpRequest();     }
          catch (e3) {  xhr = false;   }
        }
     }
  
    xhr.onreadystatechange  = function(){ 
         if(xhr.readyState  == 4 && xhr.status  == 200){

            callback&&(callback(JSON.parse(xhr.responseText),xhr))
         }
    };
    xhr.open('GET', url,  true); 
    xhr.send(null); 
}