判断元素在是否在可视区域 check element in viewport(include part of)

isElementVisible.js

// include part of the element
// 包含元素的一部分区域在可以区域
function isElementVisble(el) {
  if(!el) {
    return;
  }
  
  var viewportHeight = Math.max(document.documentElement.clientHeight, window.innerHeight || 0);
  var elPos = el.getBoundingClientRect();
  
  return ( elPos.top > 0 && (elPos.top < viewportHeight) ) ||
          (elPos.bottom > 0 && elPos.bottom < viewportHeight);
}