alert(document.cookie);     //弹出是空的

document.cookie="user=jack";

document.cookie="user=jack";
document.cookie="pass=123456";

//document.cookie="user=jack";
//document.cookie="pass=123456";
alert(document.cookie);

var oDate=new Date();
oDate.setDate(oDate.getDate()+14);    //为了得出14天以后是几号
document.cookie="user=jack;expires="+oDate;
document.cookie="pass=123456";
alert(document.cookie);

//设置cookie
function setCookie(name,value,iDay){var oDate=new Date();oDate.setDate(oDate.getDate()+iDay);    //iDay是几天过期document.cookie=name+'='+value+';expires='+oDate;
}
//读取cookie
function getCookie(name){         var arr=document.cookie.split("; ");for(var i=0;i<arr.length;i++){var arr2=arr[i].split('=');if(arr2[0]==name){return arr2[1];} }return '';
}
//删除cookie
function removeCookie(name){setCookie(name,1,-1);             //-1天过期
}

//调用
setCookie("user","lynn",13);
setCookie("password","123456",365);
setCookie("words","book",365);
alert(document.cookie);
removeCookie("password");
alert(document.cookie);

if(getCookie("pop")=="open"){alert("我是第N次打开");
}else{alert("我是第一次打开");setCookie("pop","open",1);     //1天以后过期
}

<form id="form1" action="">用户名:<input type="text" name="user" /><br />密码:<input type="password" name="pass" /><br /><input type="submit" value="登陆" />
</form><script type="text/javascript">window.onload=function(){var oForm=document.getElementById('form1'),oUser=document.getElementsByName('user')[0];oForm.onsubmit=function(){setCookie('user',oUser.value,14)}oUser.value=getCookie('user');}
</script>