To top button on JavaScript with jQuery library

to-top-button.html

<a id="bw_to_top" href="#top" style="
	position: fixed;
	left: 20px;
	bottom: 20px;
	display: block;
	opacity: 0;
" title="To top">To top</a>

<script type="text/javascript">
	var obj = $('#bw_to_top'); // object of to_top button
	var offset = 300; // offset from the current position of to_top button
	var frozen = true; // flag that show is show to_top button

	$(window).scroll(function(){
		var showed = obj.offset().top - offset > $(window).height();
        if (showed == true && frozen == true) {
			// show to_top button
			frozen = false;
			obj.stop().animate({'opacity': 1}, 800);
		} else if ( showed == false && frozen == false) {
			// hide to_top button
			frozen = true;
			obj.stop().animate({'opacity': 0}, 800);
		}
	});
</script>