元素是否在可視區域內

/** * 垂直方向是否在可視區域內 */

function isInViewY(element, offset) {
  const pageYScroll = window.pageYOffset || document.documentElement.scrollTop;
  const pageHeight = window.innerHeight || document.documentElement.clientHeight || document.body.clientHeight;
  const eleRect = element.getBoundingClientRect();

  const wt = pageYScroll;
  const wb = wt + pageHeight;
  const et = wt + eleRect.top;
  const eb = et + eleRect.height;

  return eb >= wt - offset && et <= wb + offset;
}

/** * 橫向方向是否在可視區域內 */

function isInViewX(element, offset) {
  const pageXScroll = window.pageYOffset || document.documentElement.scrollLeft;
  const pageWidth = window.innerWidth || document.documentElement.clientWidth;
  const eleRect = element.getBoundingClientRect();
  const wt = pageXScroll;
  const wb = wt + pageWidth;
  const et = wt + eleRect.left;
  const eb = et + eleRect.width;

  return eb >= wt - offset && et <= wb + offset;

}

/** * 元素是否在視口中顯示 */

function isInView(element, scrollType) {
  // 元素露出 2/3 後發送可見曝光統計
  const offset = 0;
  if (!isInViewY(element, offset)) return false;
  // 除了 x 表示橫軸滾動，其他均為y軸滾動
  if (scrollType !== 'x' || isInViewX(element, offset)) {
    return true;
  }
  return false;
}