判断元素是否在视窗内

代码如下：

const isInViewport = function (elem) {
  const bounding = elem.getBoundingClientRect()
  return (
    bounding.top >= 0 &&
    bounding.left >= 0 &&
    bounding.bottom <= (window.innerHeight || document.documentElement.clientHeight) &&
    bounding.right <= (window.innerWidth || document.documentElement.clientWidth)
  )
}