原生js实现文字无缝向上滚动效果

在网上查找到了一个实现文字无缝向上滚动效果的源码，结果发现运行有问题，于是就稍加改动，实现了js无缝向上滚动的效果。

<script>
        window.onload = roll(50);
        function roll(t) {
            var ul1 = document.getElementById("ul1");
            var ul2 = document.getElementById("ul2");
            var box = document.getElementById("broadcast");
            ul2.innerHTML = ul1.innerHTML;
            box.scrollTop = 0;
            var timer = setInterval(rollStart, t);
            box.onmouseover = function () {
                clearInterval(timer)
            }
            box.onmouseout = function () {
                timer = setInterval(rollStart, t);
            }
        }

        function rollStart() {

            if (document.getElementById("broadcast").scrollTop >= document.getElementById("ul1").scrollHeight) {
                document.getElementById("broadcast").scrollTop = 0;
            } else {
                document.getElementById("broadcast").scrollTop++;
            }
        }
    </script>