ScrollToTop animation using pure javascript (not jquery)

ScrollToTop.css

/*!
=> Plugin: scrollToTop
=> by zkreations.com | Daniel Abel 
*/
/*reset button*/  
.scrollToTop {
   outline: none;
   border: none;
   font-size: 14px;
   cursor: pointer;
}
.scrollToTop svg {
   display: block;
}
/*style*/ 
.scrollToTop {
   position: fixed;
   opacity: 0;
   visibility: hidden;
   transition: transform .3s, visibility .3s, opacity .3s, background .3s;
   transform: translateY(-2em);
}
.scrollToTop.visible {
   visibility: visible;
   opacity: 1;
   transform: translateY(0);
}

/*Personalize*/
.scrollToTop svg {
   fill: #FFFFFF;
}
.scrollToTop {
   background-color: rgba(0, 0, 0, 0.85);
   box-shadow: 0 1px 2px rgba(0,0,0,0.5), 0 1px 7px rgba(0,0,0,0.25);
   bottom: 1em;
   right: 1em;
   color: #fff;
   padding: .5em;
   border-radius: 4px;
}
.scrollToTop:hover {
   background-color: rgba(0, 0, 0, 0.95);
}

ScrollToTop.js

// Plugin: scrollToTop
// by zkreations.com | Daniel Abel
var scrollToTop = (function() {
    var showButton = 600,
        scrollSpeed = 1000; 
    function scrollTop(b) {
        function a(d) {
            c += Math.PI / (b / (d - e));
            c >= Math.PI && window.scrollTo(0, 0);
            0 !== window.scrollY && (window.scrollTo(0, Math.round(scrollTime + scrollTime * Math.cos(c))), e = d, window.requestAnimationFrame(a))
        }
        var scrollTime = window.scrollY / 2,
            c = 0,
            e = performance.now();
        window.requestAnimationFrame(a)
    }
    var scrollPosition = window.scrollY,
        scrollButton = document.getElementById("scrollToTop");
    window.addEventListener("scroll", function() {
        scrollPosition = window.scrollY;
        showButton < scrollPosition ? scrollButton.classList.add("visible") : scrollButton.classList.remove("visible")
    });
    scrollButton.onclick = function() {
        scrollTop(scrollSpeed)
    }
})();

// HTML example :
// 
// <button id="scrollToTop" class="scrollToTop">
// <svg xmlns="http://www.w3.org/2000/svg" width="35" height="35" viewBox="0 0 24 24">
//    <path d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"/>
//    <path d="M0 0h24v24H0z" fill="none"/>
// </svg>
// </button>
//
// Enjoy!