惰性函数

 1 function createXhr(){
 2     var xhr=null;
 3     if(typeof XMLHttpRequest!=‘undefined‘){
 4         xhr=new XMLHttpRequest();
 5         createXhr=function(){
 6             return new XMLHttpRequest();
 7         }
 8     }else{
 9         try{
10             xhr=new ActiveXObject(‘Msxml2.XMLHTTP‘);
11             createXhr=function(){
12                 return new ActiveXObject(‘Msxml2.XMLHTTP‘);
13             }
14         }catch(e){
15             xhr=new ActiveXObject(‘Microsoft.XMLHTTP‘);
16             createXhr=function(){
17                 return new ActiveXObject(‘Microsoft.XMLHTTP‘);
18             }
19         }
20     }
21     return xhr;
22 
23 }
24 createXhr();