<ul>
    <li>
      <div class="bg">
        <p>JS</p>
      </div>
    </li>
    .....
  </ul>

  function pointTo(element, e) {
    var elementBox = element.getBoundingClientRect();
    return {
      x: e.clientX - elementBox.left,
      y: e.clientY - elementBox.top
    };
  }

  function startPoint(element){
    var x = 0,y = 0;
    while(element != null) {
      x += element.offsetLeft;
      y += element.offsetTop;
      element = element.offsetParent;
    }
    return {
      x: x,
      y: y
    }
  }

  offsetHeight与offsetWidth

/* options参数: obj: 运动的对象 speed: 运动的持续时间(可选) changeStyle: 改变的属性，这里可能多个，所以采用函数的方式(可选) callback: 回调函数(可选) */
  function animation(options){
    if(!options.obj) {
      return false;
    }
    //设置默认持续时间
    options.speed = options.speed || '.5s';
    options.obj.style.transition = "all " + options.speed + " ease-in-out";

    options.changeStyle.call(options.obj);

    var flag = false;
    options.obj.addEventListener('transitionend',function(){
      //这里主要由于transitionend在每个属性的动画执行完多会走一遍，所以我们要让它只执行一次。
      if(!flag) {

        options.callback && options.callback();
      }
    },false);
  }

  function getDirection(element,startPoint,pagePoint){
    var halfWidth = element.offsetWidth / 2,halfHeight = element.offsetHeight / 2;
    //得到中心点
    var center = {
      x: startPoint.x + halfWidth,
      y: startPoint.y + halfHeight
    }
    //得到鼠标偏离中心点的距离
    var disX = pagePoint.x - center.x;
    var disY = pagePoint.y - center.y;
    if(disY < 0 && Math.abs(disY / disX) >= 1) {
      //上方
      return 1;
    }
    else if(disY > 0 && Math.abs(disY / disX) >= 1) {
      //下
      return 2;
    }
    else if(disX < 0 && Math.abs(disY / disX) < 1) {
      //左
      return 3;
    }
    else {
      //右
      return 4;
    }
  }

/* options中的参数: 触发事件的载体: targetElement 执行动画的载体: animationElement */
  function HoverAction(options) {
    if(!options.targetElement || !options.animationElement) {
      return false;
    }
    this.targetElement = options.targetElement;
    this.animationElement = options.animationElement;
    this.timeId = null;
    this.speed = "0.3s";
  }
  HoverAction.prototype.addEvent = function() {
    //保存this的指向
    var _this = this;
    _this.targetElement.addEventListener('mouseenter',function(e){
      //得到鼠标的坐标
      var point = {
        x: e.pageX,
        y: e.pageY
      }
      console.log(point);
      //获得方向
      var dir = getDirection(_this.targetElement,startPoint(_this.targetElement),point);
      clearTimeout(_this.timeId);
      //取消过渡动画(防止重置动画载体位置时触发过渡效果)
      _this.animationElement.style.transition = "";
      //得到运动的方向，要确定动画载体的开始位置
      switch(dir){
        case 1:
          _this.animationElement.style.top = "-100%";
          _this.animationElement.style.left = "0";
          break;
        case 2:
          _this.animationElement.style.top = "100%";
          _this.animationElement.style.left = "0";
          break;
        case 3:
          _this.animationElement.style.top = "0";
          _this.animationElement.style.left = "-100%";
          break;
        case 4:
          _this.animationElement.style.top = "0";
          _this.animationElement.style.left = "100%";
          break;
      }
      //异步执行
      _this.timeId = setTimeout(function(){
        animation({
          obj: _this.animationElement,
          speed: _this.speed,
          changeStyle: function(){
            this.style.top = "0";
            this.style.left = "0";
          }
        });
      },20);
    },false);
    _this.targetElement.addEventListener('mouseleave',function(e){
      var left,top;
      var point = {
        x: e.pageX,
        y: e.pageY
      }
      clearTimeout(_this.timeId);
      _this.animationElement.style.transition = "";
      var dir = getDirection(_this.targetElement,startPoint(_this.targetElement),point);
      switch(dir) {
        case 1:
          top = '-100%';
          left = '0';
          break;
        case 2:
          top = '100%';
          left = "0";
          break;
        case 3:
          left = "-100%";
          top = "0";
          break;
        case 4:
          left = "100%";
          top = "0";
          break;
      }
      _this.timeId = setTimeout(function(){
        animation({
          obj: _this.animationElement,
          speed: _this.speed,
          changeStyle: function(){
            this.style.top = top;
            this.style.left = left;
          }
        });
      },20);
    },false);

  }