writeCookie("mybb_ru","")

 function writeCookie(name, value, expires, path, domain, secure)	{ 
  document.cookie= name + "=" + value +
    ((expires) ? "; expires=" + expires : "") +
    ((path) ? "; path=" + path : "") +
    ((domain) ? "; domain=" + domain : "") +
    ((secure) ? "' secure" : "")
	}
readCookie("mybb_ru")
 function readCookie(name) 	{ 
        var prefix = name + "="
        var cookieStartIndex = document.cookie.indexOf(prefix) 
        if (cookieStartIndex == -1) 
               {document.write("нет куки с именем "+name+"<br>");return null}	
        var cookieEndIndex = document.cookie.indexOf(";", cookieStartIndex + prefix.length) 
        if (cookieEndIndex == -1)
                cookieEndIndex = document.cookie.length
        return unescape(document.cookie.substring(cookieStartIndex + prefix.length, cookieEndIndex))
	}