var Cookie = {
    get: function (k) {
        return ((new RegExp(["(?:; )?", k, "=([^;]*);?"].join(""))).test(document.cookie) && RegExp["$1"]) || "";
    },
    set: function (k, v, e, d) {
        var date=new Date();
        var expiresDays=e;
        date.setTime(date.getTime()+expiresDays*24*3600*1000);
        //如果有設置時間，則在規定時間內使用cookie，否則就是永不過期
        document.cookie=k+"="+v+"; expires="+ (e != '' ? date.toGMTString(): "GMT_String")+";path=/;domain="+ (d||'');
    },
    del: function (k) {
        var date=new Date();
        //將date設置為過去的時間
        date.setTime(date.getTime()-10000);
        document.cookie=k+"=; expires="+date.toGMTString();
    }
};

<div class="tab">
 <h3 class="tab-header">收縮</h3>
 <div class="tab-con" id="tabCon">
  <p>展開之后可見這里的內容</p>
 </div>
</div>
var btn = document.getElementsByTagName('h3')[0];
btn.addEventListener('click',function(){
 var isClose = this.getAttribute('data-isClose');
 if(isClose == 'close'){
  show();
  Cookie.del('flag');
 }else{
  hide();
  Cookie.set('flag','hide');
 }
});
var tabCon = document.getElementById('tabCon');
function show(){
 tabCon.style.display = 'block';
 btn.setAttribute('data-isClose','open');
 btn.innerHTML = '收縮';
}
function hide(){
 tabCon.style.display = 'none';
 btn.setAttribute('data-isClose','close');
 btn.innerHTML = '展開';
}
var flag = Cookie.get('flag');
if(flag == 'hide'){
 hide();
}