Botón back to top con Jquery

El botón “back to top” aparece una vez se empieza a hacer scroll y desaparece cuando está el scroll está en la parte superior.

Añadiremos la siguiente linea en nuestro html, justo antes de cerrar el body.

<a href="#0" class="cd-top">Top</a>

CSS

.cd-top {
    display: inline-block;
    height: 40px;
    width: 40px;
    position: fixed;
    bottom: 40px;
    right: 10px;
    box-shadow: 0 0 10px rgba(0, 0, 0, 0.05);
    /* image replacement properties */
    overflow: hidden;
    text-indent: 100%;
    white-space: nowrap;
    background: rgba(166, 44, 39, 0.8) url(../img/cd-top-arrow.svg) no-repeat center 50%;
    visibility: hidden;
    opacity: 0;
    -webkit-transition: opacity .3s 0s, visibility 0s .3s;
    -moz-transition: opacity .3s 0s, visibility 0s .3s;
    transition: opacity .3s 0s, visibility 0s .3s;
}
.cd-top.cd-is-visible, .cd-top.cd-fade-out, .no-touch .cd-top:hover {
    -webkit-transition: opacity .3s 0s, visibility 0s 0s;
    -moz-transition: opacity .3s 0s, visibility 0s 0s;
    transition: opacity .3s 0s, visibility 0s 0s;
}
.cd-top.cd-is-visible {
    /* the button becomes visible */
    visibility: visible;
    opacity: 1;
}
.cd-top.cd-fade-out {
    /* if the user keeps scrolling down, the button is out of focus and becomes less visible */
    opacity: .5;
}
.no-touch .cd-top:hover {
    background-color: #e86256;
    opacity: 1;
}
@media only screen and (min-width: 768px) {
    .cd-top {
        right: 20px;
        bottom: 20px;
    }
}
@media only screen and (min-width: 1024px) {
    .cd-top {
        height: 60px;
        width: 60px;
        right: 30px;
        bottom: 30px;
    }
}

Y el código javascript (necesita de la librería jQuery)

$(document).ready(function(){
var offset = 300,
offset_opacity = 1800,
scroll_top_duration = 700,
$back_to_top = $('.cd-top');
$(window).scroll(function(){
( $(this).scrollTop() > offset ) ? $back_to_top.addClass('cd-is-visible') : $back_to_top.removeClass('cd-is-visible cd-fade-out');
if( $(this).scrollTop() > offset_opacity ) {
$back_to_top.addClass('cd-fade-out');
}
});
$back_to_top.on('click', function(event){
event.preventDefault();
$('body,html').animate({
scrollTop: 0 ,
}, scroll_top_duration
);
});
});

Y la imagen: cd-top-arrow