https://www.congresojuridicoabogaciademalaga.es/land/assets/plugins/backtotop.js

1$(document).ready(function($){
2    // browser window scroll (in pixels) after which the "back to top" link is shown
3    var offset = 300,
4        //browser window scroll (in pixels) after which the "back to top" link opacity is reduced
5        offset_opacity = 1200,
6        //duration of the top scrolling animation (in ms)
7        scroll_top_duration = 700,
8        //grab the "back to top" link
9        $back_to_top = $('.js-back-to-top');
10
11    //hide or show the "back to top" link
12    $(window).scroll(function(){
13        ( $(this).scrollTop() > offset ) ? $back_to_top.addClass('back-to-top-is-visible') : $back_to_top.removeClass('back-to-top-is-visible back-to-top-fade-out');
14        if( $(this).scrollTop() > offset_opacity ) {
15            $back_to_top.addClass('back-to-top-fade-out');
16        }
17    });
18
19    //smooth scroll to top
20    $back_to_top.on('click', function(event){
21        event.preventDefault();
22        $('body,html').animate({
23            scrollTop: 0 ,
24            }, scroll_top_duration
25        );
26    });
27});