如何用JS获得Table中TH的宽度

        <table cellspacing="0" rules="all" border="1" id="Table1" style="border-collapse: collapse;">
            <tr style="width: 20px;">
                <th scope="col">
                    学生班级</th>
                <th scope="col">
                    学生姓名</th>
                <th scope="col">
                    语文</th>
                <th scope="col">
                    数学</th>
                <th scope="col">
                    英语</th>
                <th scope="col">
                    计算机</th>
            </tr>
            <tr style="width: 20px;">
                <td>
                    班级0</td>
                <td>
                    0</td>
                <td>
                    72.62</td>
                <td>
                    81.73</td>
                <td>
                    76.8</td>
                <td>
                    55.82</td>
            </tr>
            <tr style="width: 20px;">
                <td>
                    班级1</td>
                <td>
                    1</td>
                <td>
                    47.51</td>
                <td>
                    26.1</td>
                <td>
                    68.84</td>
                <td>
                    63.85</td>
            </tr>
            <tr style="width: 20px;">
                <td>
                    班级2</td>
                <td>
                    2</td>
                <td>
                    22.4</td>
                <td>
                    70.47</td>
                <td>
                    60.87</td>
                <td>
                    71.89</td>
            </tr>
            <tr style="width: 20px;">
                <td>
                    班级3</td>
                <td>
                    3</td>
                <td>
                    97.29</td>
                <td>
                    14.84</td>
                <td>
                    52.9</td>
                <td>
                    79.92</td>
            </tr>
            <tr style="width: 20px;">
                <td>
                    班级4</td>
                <td>
                    4</td>
                <td>
                    72.18</td>
                <td>
                    59.21</td>
                <td>
                    44.94</td>
                <td>
                    87.95</td>
            </tr>
            <tr style="width: 20px;">
                <td>
                    班级5</td>
                <td>
                    5</td>
                <td>
                    47.07</td>
                <td>
                    3.58</td>
                <td>
                    36.97</td>
                <td>
                    95.99</td>
            </tr>
            <tr style="width: 20px;">
                <td>
                    班级6</td>
                <td>
                    6</td>
                <td>
                    21.96</td>
                <td>
                    47.95</td>
                <td>
                    29.01</td>
                <td>
                    4.02</td>
            </tr>
            <tr style="width: 20px;">
                <td>
                    班级7</td>
                <td>
                    7</td>
                <td>
                    96.85</td>
                <td>
                    92.32</td>
                <td>
                    21.04</td>
                <td>
                    12.06</td>
            </tr>
            <tr style="width: 20px;">
                <td>
                    班级8</td>
                <td>
                    8</td>
                <td>
                    71.74</td>
                <td>
                    36.68</td>
                <td>
                    13.08</td>
                <td>
                    20.09</td>
            </tr>
            <tr style="width: 20px;">
                <td>
                    班级9</td>
                <td>
                    9</td>
                <td>
                    46.63</td>
                <td>
                    81.05</td>
                <td>
                    5.11</td>
                <td>
                    28.13</td>
            </tr>
        </table>_ths = document.getElementById('GridView2').getElementsByTagName('TH');
_ths[0].width //无法获得
_ths[0].style.width //无法获得