js 实现缓动动画

利用定时器来控制元素的offsetLeft的值，

offsetLeft = 开始位置 + （最终位置 - 开始位置）* 缓动系数

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>

  <style>
    * {
      margin: 0;
      padding: 0;
      border: none;
      list-style: none;
    }

    body {
      background-color: pink;
    }

    #nav {
      width: 900px;
      height: 63px;
      background: url("images/doubleOne.png") no-repeat right center #fff;
      margin: 0 auto;
      margin-top: 50px;
      border-radius: 5px;
      position: relative;
    }

    #nav ul {
      line-height: 70px;
    }

    #nav ul li {
      float: left;
      height: 63px;
      width: 88px;
      text-align: center;
      cursor: pointer;
      position: relative;
    }

    #t_mail {
      width: 88px;
      height: 63px;
      background: url("images/tMall.png") no-repeat;
      position: absolute;
    }
  </style>
</head>
<body>
  <nav id="nav">
    <span id="t_mail"></span>
      <ul>
        <li>双11狂欢</li>
        <li>服装会场</li>
        <li>数码家电</li>
        <li>母婴童装</li>
        <li>手机会场</li>
        <li>美妆会场</li>
        <li>家居建材</li>
        <li>进口会场</li>
        <li>飞猪旅行</li>
      </ul>
  </nav>

  <script>
    window.onload = function () {
      var nav = $('nav');
      var t_mall = nav.children[0];
      var ul = nav.children[1];
      var allLis = ul.children;
      var beginX = 0;

      for (var i = 0; i < allLis.length; i++) {
        var li = allLis[i];
        li.onmouseover = function () {
          end = this.offsetLeft;
        }

        li.onmousedown = function () {
          beginX = this.offsetLeft;
        }

        li.onmouseout = function () {
          end = beginX;
        }
      }

      var begin = 0, end = 0;
      setInterval(function () {
        begin = begin + (end - begin) / 10;
        t_mall.style.left = begin + 'px';
      }, 10)

      function $(id) {
        return typeof id ? document.getElementById(id) : null;
      }
    }
  </script>
</body>
</html>