动态添加div中，需要进行添加id，class，style等属性

  var div = document.createElement('div');
  div.id = 'mask_div1';
  div.className = 'mask';
  div.innerText = 'abc'; //给div赋文本值
  div.appendChild(document.createTextNode("test"));
  div.style.position = "absolute";
  div.style.left =  '20px';
  div.style.top =  '20px';
  div.style.overflow = "hidden";
  div.style.zIndex = "9999";
  div.style.opacity = 0.3;
  document.body.appendChild(div);