How to change ID to Class in Javascript

[HakuAnime]

What do you do to change a javascript from a ID to a Class? I tried to do it on my own but with no luck, this is what I ended up with. The original code was to show or hide an element and remember the users selection.HTML:\[code\]<div class="popup" style="display:none"> <img src="http://stackoverflow.com/questions/9833429/image-url.jpg" alt="alt text" title="title text"></div>\[/code\]JAVASCRIPT:\[code\]function setCookie (name, value, expires, path, domain, secure) { document.cookie = name + "=" + escape(value) + ((expires) ? "; expires=" + expires : "") + ((path) ? "; path=" + path : "") + ((domain) ? "; domain=" + domain : "") + ((secure) ? "; secure" : "");}function getCookie (name) { var cookie = " " + document.cookie; var search = " " + name + "="; var setStr = null; var offset = 0; var end = 0; if (cookie.length > 0) { offset = cookie.indexOf(search); if (offset != -1) { offset += search.length; end = cookie.indexOf(";", offset); if (end == -1) { end = cookie.length; } setStr = unescape(cookie.substring(offset, end)); } } if (setStr == 'false') { setStr = false; } if (setStr == 'true') { setStr = true; } if (setStr == 'null') { setStr = null; } return(setStr);}function hidePopup() { setCookie('popup_state', false); document.getElementsByClassName('popup').style.display = 'none';}function showPopup() { setCookie('popup_state', null); document.getElementsByClassName('popup').style.display = 'block';}function checkPopup() { if (getCookie('popup_state') == null) { // if popup was not closed document.getElementsByClassName('popup').style.display = 'block'; }}\[/code\]