那位高手帮我讲一下这段js代码

#ajah.jsvar  Ajah=function(url,varname,handleSuccess,handleFailure){ 
        /** 
        * handleSuccess,handleFailure must be functions 
        * */ 
        script = document.createElement("script"); 
        script.src=url; 
        var handler = function(str) 
        { 
                handleSuccess(str); 
        } 
        script.onload = function() 
        { 
                var json=eval(varname); 
                handler(json); 
        } 
        if(window.ie) 
        { 
                script.onreadystatechange = function() 
                { 
                        if(script.readyState=='complete'||script.readyState== 'loaded') 
                        { 
                                var json=eval(varname); 
                                if(typeof json != 'undefined') 
                                { 
                                        handler(json); 
                                } 
                        } 
                } 
        } 
        document.body.appendChild(script); 
}而在网页中应这样调用:#demo.html<pre> 
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN"> 
<html> 
<head> 
  <meta name="generator" content= 
  "HTML Tidy for Linux (vers 1 September 2005), see www.w3.org"> 
  
  <title></title> 
        <script src="mootools.js"></script> 
        <script src="ajah.js"></script> 
</head> 
  
<body> 
<script> 
var ajah=new Ajah("data.js","json198",function(str){ 
        console.debug("returned:"); 
        console.debug(str); 
}, 
function(str){}); 
</script> 
</body> 
</html> 
</pre>
被调用的数据文件如下#data.jsvar json198="hello,this is json198";
funciton Ajah(url,varname,handleSuccess,handleFailure){…}
Ajah这个构造函数调用四个参数:
url:远端地址
varname:远端返回数据的变量名
handleSuccess:加载完毕后加载的函数
handleFailure:暂时没用上