谁能帮我细解一下javascript？？

function checkusername(username)
{
var  options={  
   method:'get',  
   parameters:"Action=checkusername&username="+username,  
   onComplete:function(transport)
{  
var returnvalue=transport.responseText;

if (returnvalue.indexOf("??")>-1)
    document.getElementById("div_content").innerHTML="Error";
else
document.getElementById("div_content").innerHTML=returnvalue;
}  
   }; 
new  Ajax.Request('register.aspx?no-cache='+Math.random(),options);
}