设置cookie
function setCookie(cname,cvalue,exdays)
{
  var d = new Date();
  d.setTime(d.getTime()+(exdays*24*60*60*1000));
  var expires = "expires="+d.toGMTString();
  ocument.cookie = cname + "=" + cvalue + "; " + expires;
}
 获取cookie
function getCookie(cname)
{
  var name = cname + "=";
  var ca = document.cookie.split(‘;‘);
  for(var i=0; i<ca.length; i++) 
  {
    var c = ca[i].trim();
    if (c.indexOf(name)==0) return c.substring(name.length,c.length);
  }
  return "";
}
 检查cookie
function checkCookie()
{
  var user=getCookie("username");
  if (user!="")
  {
    alert("Welcome again " + user);
  }
  else 
  {
    user = prompt("Please enter your name:","");
    if (user!="" && user!=null)
    {
      setCookie("username",user,365);
    }
  }
}