Make button go to top

<script>
$(document).on("scroll",function(){
console.log($(this).scrollTop());
if($(this).scrollTop()>100){
$("#top_b").fadeIn();
}else{
$("#top_b").fadeOut();
}
});
$(document).ready(function() {
$('#top_b').on("click",function(){
console.log($.now());
$('html, body').animate({scrollTop : 0},500);
return false;
});
});

</script>
<style>
#top_b{
position:fixed;
bottom:20px;right:20px;
display:none;
}
</style>
<a href="#" id="top_b">Top</a>