此代码内容为自动隐藏的弹性返回顶部JS代码，超出首屏范围，即会出现TOP按钮，否则自动隐藏。有弹性返回顶部的动画效果。-The content of this code to automatically hide the flexibility to return to the top of the JS code, beyond the scope of the first screen that appears TOP button, or automatically hide. Flexibility to return to the top of the animation.