【无缝滚动】为什么向右滚动不能实现条件语句？

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>左右无缝滚动</title>
    <style>
        * {
            margin: 0%;
            padding: 0%;
        }

        #div1 {
            position: relative;
            border: 1px black solid;
            width: 680px;
            height: 132px;
            margin: 10px auto;
        }

        #div1 ul {
            position: absolute;
            left: 0;
        }

        #div1 ul li {
            list-style: none;
            float: left;
            width: 150px;
            height: 112px;
            padding: 10px;
        }

        #div1 ul li img {
            width: 150px;
        }
    </style>
    <script>
        window.onload = function () {
            var oDiv = document.getElementById('div1');
            var oUl = oDiv.getElementsByTagName('ul')[0];
            var aLi = oUl.getElementsByTagName('li');
            var iSpeed = 10;
            var timer = null;

            oUl.innerHTML += oUl.innerHTML; //自我复制一份

            oUl.style.width = aLi.length * aLi[0].offsetWidth + 'px';

            timer = setInterval(function () {
                if (oUl.offsetLeft < -oUl.offsetWidth / 2) { //当此整体刚好出去一般的时候触发条件：此时整个图层还原原来的位置
                    oUl.style.left = 0;
                } else if (oUl.offsetLeft > 0) {//这里是当向右滚动时，返回
                    oUl.style.left = -oUl.offsetWidth / 2;
                }
                oUl.style.left = oUl.offsetLeft + iSpeed + 'px';
            }, 30)
        }
    </script>
</head>

<body>
    <div id="div1">
        <ul>
            <li><img src="image/1.jpg" alt="a" /></li>
            <li><img src="image/2.jpg" alt="a" /></li>
            <li><img src="image/3.jpg" alt="a" /></li>
            <li><img src="image/4.jpg" alt="a" /></li>
        </ul>
    </div>
</body>

</html>
问题已解决：oUl.style.left = -oUl.offsetWidth / 2 + 'px';