判断dom原始是否在可视区域内

isElementInViewport (el, offset = 0) {
  const box = el.getBoundingClientRect(),
    top = (box.top >= 0),
    left = (box.left >= 0),
    bottom = (box.bottom <= (window.innerHeight || document.documentElement.clientHeight) + offset),
    right = (box.right <= (window.innerWidth || document.documentElement.clientWidth) + offset)
  return (top && left && bottom && right)
}