js改变cookie的值

//点击退出方法
function logout(){
 
    var myDate=new Date();    
var data=document.cookie; 
    var dataArray=data.split("=");
    var c_name=dataArray[0];
    setCookie(c_name, "", -1);
   
   }
 //改变cookie的值    
 function setCookie(c_name, value, expiredays){
    var date = new Date();
    date.setTime(date.getTime());
    expires = "; expires="+date.toGMTString();
    document.cookie = c_name+"="+escape(value)+expires+"; path=/"; 
   }