<p>Hello</p>
<p></p>

$(window).scroll(function () {
  $('p:last').text('scrollTop:' + win.scrollTop());
});

<p>Hello</p>
<p>2nd Paragraph</p>

var p = $('p:last');
var offset = p.offset().top;
p.text('top: ' + offset);

완성화면 탑버튼을 만들어보자

완성화면