小方块向左向右运动

代码
<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        #box {
            width: 100px;
            height: 100px;
            background: darkmagenta;
            position: absolute;
            left: 0;
            top: 30px;
        }
    </style>
</head>

<body>
    <button id='right'>向右走</button>
    <button id='left'>向左走</button>
    <div id="box"></div>

    <script>
        var right = document.getElementById('right');
        var left = document.getElementById('left');
        var box = document.getElementById('box');
        var timer = null;
        // 向右走
        right.onclick = function () {
            run(box,'left',5,500)
        }

        // 向左走
        left.onclick = function () {
            run(box,'left',10,0)
        }

        function run(ele,attr,step,target) {
            step = target>parseInt(getStyle(ele, attr))? step :-step;
            // if( target>parseInt(getStyle(ele, attr))){
            //     step = step;
            // }else{
            //     step = -step;
            // }
            clearInterval(timer);
            timer = setInterval(function () {
                var dis = parseInt(getStyle(ele, attr)) + step;
                if (dis <= target && step < 0 || dis >= target && step > 0) {
                    dis = target;
                    clearInterval(timer);
                }
                ele.style[attr] = dis + 'px';
            }, 50)
        }




        // 获取元素样式
        function getStyle(ele, attr) {
            if (window.getComputedStyle) {
                return getComputedStyle(ele)[attr];
            } else {
                return ele.currentStyle[attr];
            }
        }
    </script>
</body>

</html>
小方块向左向右运动

效果
