Fixed button scroll to top appear on scrolling from top

gistfile1.txt

document.addEventListener('DOMContentLoaded', function() {
    function throttle(fn, threshhold, scope) {
        threshhold || (threshhold = 250);
        var last,
            deferTimer;
        return function () {
            var context = scope || this;

            var now = +new Date,
                args = arguments;
            if (last && now < last + threshhold) {
                // hold on to it
                clearTimeout(deferTimer);
                deferTimer = setTimeout(function () {
                    last = now;
                    fn.apply(context, args);
                }, threshhold);
            } else {
                last = now;
                fn.apply(context, args);
            }
        };
    }
    function checkScrollPositionForButton(pos){
        if (pos>100){
            jQuery('#fixed-button-to-top').removeClass('hidden');
        } else {
            jQuery('#fixed-button-to-top').addClass('hidden');
        }
    }

    checkScrollPositionForButton(jQuery(window).scrollTop());

    jQuery(window).scroll(throttle(function (event) {
        checkScrollPositionForButton(jQuery(window).scrollTop());
    }, 150));

    jQuery('#fixed-button-to-top').on('click', function(e){
        e.preventDefault();
        window.scroll({
            top: 0,
            left: 0,
            behavior: 'smooth'
        });
    })
})

<a href="#" id="fixed-button-to-top" style="border: 1px solid black;padding: 20px;position: fixed;z-index:1000;bottom:50px;right:0;">Scrolltop</a>