+ 7
var op = parseInt(Joey.innerHTML, 10);
"innerHTML" returns text (a string), so the 5 you added was converted to text, too. Converting op to a number like I did will work :)
+ 4
Joey.innerHTML=parseInt(op)+5;
+ 1
<body>
<div id="Chandler"> 8</div>
</body>
<script>
var Joey = document.getElementById ("Chandler");
var op=Joey.innerHTML;
Joey.innerHTML=parseInt(op)+5;
</script>
+ 1
Joey.innerHTML=+op+5;