1.按id取得对象
function $(id){
     return document.getElementById(id);
}

2.创建XmlHttpRequest对象
function createXmlHttpRequest (){
     try{return new ActiveXObject("Msxml2.XMLHTTP");}catch(e){};
     try{return new ActiveXObject("Microsoft.XMLHTTP");}catch(e){};
     try{return new XMLHttpRequest();}catch(e){};
     alert("XmlHttpRequest not supported!");
     return null;
}