自己实现一个insertAfter方法

 function customInsertAfter: function (newelement, targetelement) {
      let parentelement = targetelement.parentNode;
      if (parentelement.lastChild == targetelement) {
        parentelement.appendChild(newelement);
      }
      else {
        parentelement.insertBefore(newelement, targetelement.nextSilbing);
      }
    }