function copy(){
    var tb = document.getElementById('table');
    var tr = document.querySelectorAll("#table tr:last-of-type td").value;
    alert(tr);
    var copyalbe = tr;
    var input = document.createElement("input");
    input.value = copyalbe;
    document.body.appendChild(input);
    input.select();
    document.execCommand("copy");
    input.style.display = "none";
    alert("复制成功");
  }

<table id="table" width="200" border="1" cellspacing="0">
    <tr id="cow1">
      <td>123</td>
      <td>111</td>
    </tr>
    <tr id="cow2">
      <td>123</td>
      <td>111</td>
    </tr>
    <tr id="cow3">
      <td class="copyalbe">123</td>
      <td class="copyalbe">111</td>
    </tr>
  </table>
  <input id="btn" type="button" value="复制" onclick="copy()">

我想要获取table中最后一个tr里的两个td的值并复制，但不知道如何获取值，有老哥们指教下吗？