1. <script src="https://code.jquery.com/jquery-1.10.2.js"></script>
2. 
   
3. <p class='first'>p1</p>
4. <p class='second'>p2</p>
5. <p class='third'>p3</p>
6. 
   
7. <script>
8. $('p').after(function(i,v) {
9.   return "<div><b>" + $(this).className + "</b></div>";
10. });
11. </script>

Output

1. p1
2. undefined
3. p2
4. undefined
5. p3
6. undefined

Why I can't use $(this) here? Why I must use 'this'?