jquery 数据表 "bLengthChange": 正确，不显示长度菜单
jquery datatable "bLengthChange": true, not showing lengthmenu
您好，我设置了一个长度菜单，我的数据表版本是 DataTables 1.10.13
"dom": '<f<t><"#df"<"pull-left" i><"pull-right"p><"pull-right"l>>>',
"lengthChange": true,
"lengthMenu": [[10, 20, 30, -1], [10, 20, 30, "All"]],
但是没有显示长度菜单，我用火检查，但是发现 .dataTables_length 没有更改为 display: none;。根据我的理解，我认为这个 class 由 "lengthChange": true,，那为什么它不起作用？请帮忙。
.dataTables_length {
display: none;
}
https://jsfiddle.net/4z3ahj70/
JS
$(document).ready(function() {
$('#example').DataTable( {
"dom": '<f<t><"#df"<"pull-left" i><"pull-right"p><"pull-right"l>>>',
"lengthChange": true,
"lengthMenu": [[10, 20, 30, -1], [10, 20, 30, "All"]],
});
});
HTML
<table id="example" class="display" cellspacing="0" width="100%">
<thead>
<tr>
<th>Name</th>
<th>Position</th>
<th>Office</th>
<th>Age</th>
<th>Start date</th>
<th>Salary</th>
</tr>
</thead>
<tfoot>
<tr>
<th>Name</th>
<th>Position</th>
<th>Office</th>
<th>Age</th>
<th>Start date</th>
<th>Salary</th>
</tr>
</tfoot>
<tbody>
<tr>
<td>Tiger Nixon</td>
<td>System Architect</td>
<td>Edinburgh</td>
<td>61</td>
<td>2011/04/25</td>
<td>0,800</td>
</tr>
<tr>
<td>Garrett Winters</td>
<td>Accountant</td>
<td>Tokyo</td>
<td>63</td>
<td>2011/07/25</td>
<td>0,750</td>
</tr>
<tr>
<td>Ashton Cox</td>
<td>Junior Technical Author</td>
<td>San Francisco</td>
<td>66</td>
<td>2009/01/12</td>
<td>,000</td>
</tr>
<tr>
<td>Cedric Kelly</td>
<td>Senior Javascript Developer</td>
<td>Edinburgh</td>
<td>22</td>
<td>2012/03/29</td>
<td>3,060</td>
</tr>
<tr>
<td>Airi Satou</td>
<td>Accountant</td>
<td>Tokyo</td>
<td>33</td>
<td>2008/11/28</td>
<td>2,700</td>
</tr>
<tr>
<td>Brielle Williamson</td>
<td>Integration Specialist</td>
<td>New York</td>
<td>61</td>
<td>2012/12/02</td>
<td>2,000</td>
</tr>
<tr>
<td>Herrod Chandler</td>
<td>Sales Assistant</td>
<td>San Francisco</td>
<td>59</td>
<td>2012/08/06</td>
<td>7,500</td>
</tr>
<tr>
<td>Rhona Davidson</td>
<td>Integration Specialist</td>
<td>Tokyo</td>
<td>55</td>
<td>2010/10/14</td>
<td>7,900</td>
</tr>
<tr>
<td>Colleen Hurst</td>
<td>Javascript Developer</td>
<td>San Francisco</td>
<td>39</td>
<td>2009/09/15</td>
<td>5,500</td>
</tr>
<tr>
<td>Sonya Frost</td>
<td>Software Engineer</td>
<td>Edinburgh</td>
<td>23</td>
<td>2008/12/13</td>
<td>3,600</td>
</tr>
<tr>
<td>Jena Gaines</td>
<td>Office Manager</td>
<td>London</td>
<td>30</td>
<td>2008/12/19</td>
<td>,560</td>
</tr>
<tr>
<td>Quinn Flynn</td>
<td>Support Lead</td>
<td>Edinburgh</td>
<td>22</td>
<td>2013/03/03</td>
<td>2,000</td>
</tr>
<tr>
<td>Charde Marshall</td>
<td>Regional Director</td>
<td>San Francisco</td>
<td>36</td>
<td>2008/10/16</td>
<td>0,600</td>
</tr>
<tr>
<td>Haley Kennedy</td>
<td>Senior Marketing Designer</td>
<td>London</td>
<td>43</td>
<td>2012/12/18</td>
<td>3,500</td>
</tr>
<tr>
<td>Tatyana Fitzpatrick</td>
<td>Regional Director</td>
<td>London</td>
<td>19</td>
<td>2010/03/17</td>
<td>5,750</td>
</tr>
<tr>
<td>Michael Silva</td>
<td>Marketing Designer</td>
<td>London</td>
<td>66</td>
<td>2012/11/27</td>
<td>8,500</td>
</tr>
<tr>
<td>Paul Byrd</td>
<td>Chief Financial Officer (CFO)</td>
<td>New York</td>
<td>64</td>
<td>2010/06/09</td>
<td>5,000</td>
</tr>
<tr>
<td>Gloria Little</td>
<td>Systems Administrator</td>
<td>New York</td>
<td>59</td>
<td>2009/04/10</td>
<td>7,500</td>
</tr>
<tr>
<td>Bradley Greer</td>
<td>Software Engineer</td>
<td>London</td>
<td>41</td>
<td>2012/10/13</td>
<td>2,000</td>
</tr>
<tr>
<td>Dai Rios</td>
<td>Personnel Lead</td>
<td>Edinburgh</td>
<td>35</td>
<td>2012/09/26</td>
<td>7,500</td>
</tr>
<tr>
<td>Jenette Caldwell</td>
<td>Development Lead</td>
<td>New York</td>
<td>30</td>
<td>2011/09/03</td>
<td>5,000</td>
</tr>
<tr>
<td>Yuri Berry</td>
<td>Chief Marketing Officer (CMO)</td>
<td>New York</td>
<td>40</td>
<td>2009/06/25</td>
<td>5,000</td>
</tr>
<tr>
<td>Caesar Vance</td>
<td>Pre-Sales Support</td>
<td>New York</td>
<td>21</td>
<td>2011/12/12</td>
<td>6,450</td>
</tr>
<tr>
<td>Doris Wilder</td>
<td>Sales Assistant</td>
<td>Sidney</td>
<td>23</td>
<td>2010/09/20</td>
<td>,600</td>
</tr>
<tr>
<td>Angelica Ramos</td>
<td>Chief Executive Officer (CEO)</td>
<td>London</td>
<td>47</td>
<td>2009/10/09</td>
<td>,200,000</td>
</tr>
<tr>
<td>Gavin Joyce</td>
<td>Developer</td>
<td>Edinburgh</td>
<td>42</td>
<td>2010/12/22</td>
<td>,575</td>
</tr>
<tr>
<td>Jennifer Chang</td>
<td>Regional Director</td>
<td>Singapore</td>
<td>28</td>
<td>2010/11/14</td>
<td>7,650</td>
</tr>
<tr>
<td>Brenden Wagner</td>
<td>Software Engineer</td>
<td>San Francisco</td>
<td>28</td>
<td>2011/06/07</td>
<td>6,850</td>
</tr>
<tr>
<td>Fiona Green</td>
<td>Chief Operating Officer (COO)</td>
<td>San Francisco</td>
<td>48</td>
<td>2010/03/11</td>
<td>0,000</td>
</tr>
<tr>
<td>Shou Itou</td>
<td>Regional Marketing</td>
<td>Tokyo</td>
<td>20</td>
<td>2011/08/14</td>
<td>3,000</td>
</tr>
<tr>
<td>Michelle House</td>
<td>Integration Specialist</td>
<td>Sidney</td>
<td>37</td>
<td>2011/06/02</td>
<td>,400</td>
</tr>
<tr>
<td>Suki Burks</td>
<td>Developer</td>
<td>London</td>
<td>53</td>
<td>2009/10/22</td>
<td>4,500</td>
</tr>
<tr>
<td>Prescott Bartlett</td>
<td>Technical Author</td>
<td>London</td>
<td>27</td>
<td>2011/05/07</td>
<td>5,000</td>
</tr>
<tr>
<td>Gavin Cortez</td>
<td>Team Leader</td>
<td>San Francisco</td>
<td>22</td>
<td>2008/10/26</td>
<td>5,500</td>
</tr>
<tr>
<td>Martena Mccray</td>
<td>Post-Sales support</td>
<td>Edinburgh</td>
<td>46</td>
<td>2011/03/09</td>
<td>4,050</td>
</tr>
<tr>
<td>Unity Butler</td>
<td>Marketing Designer</td>
<td>San Francisco</td>
<td>47</td>
<td>2009/12/09</td>
<td>,675</td>
</tr>
<tr>
<td>Howard Hatfield</td>
<td>Office Manager</td>
<td>San Francisco</td>
<td>51</td>
<td>2008/12/16</td>
<td>4,500</td>
</tr>
<tr>
<td>Hope Fuentes</td>
<td>Secretary</td>
<td>San Francisco</td>
<td>41</td>
<td>2010/02/12</td>
<td>9,850</td>
</tr>
<tr>
<td>Vivian Harrell</td>
<td>Financial Controller</td>
<td>San Francisco</td>
<td>62</td>
<td>2009/02/14</td>
<td>2,500</td>
</tr>
<tr>
<td>Timothy Mooney</td>
<td>Office Manager</td>
<td>London</td>
<td>37</td>
<td>2008/12/11</td>
<td>6,200</td>
</tr>
<tr>
<td>Jackson Bradshaw</td>
<td>Director</td>
<td>New York</td>
<td>65</td>
<td>2008/09/26</td>
<td>5,750</td>
</tr>
<tr>
<td>Olivia Liang</td>
<td>Support Engineer</td>
<td>Singapore</td>
<td>64</td>
<td>2011/02/03</td>
<td>4,500</td>
</tr>
<tr>
<td>Bruno Nash</td>
<td>Software Engineer</td>
<td>London</td>
<td>38</td>
<td>2011/05/03</td>
<td>3,500</td>
</tr>
<tr>
<td>Sakura Yamamoto</td>
<td>Support Engineer</td>
<td>Tokyo</td>
<td>37</td>
<td>2009/08/19</td>
<td>9,575</td>
</tr>
<tr>
<td>Thor Walton</td>
<td>Developer</td>
<td>New York</td>
<td>61</td>
<td>2013/08/11</td>
<td>,540</td>
</tr>
<tr>
<td>Finn Camacho</td>
<td>Support Engineer</td>
<td>San Francisco</td>
<td>47</td>
<td>2009/07/07</td>
<td>,500</td>
</tr>
<tr>
<td>Serge Baldwin</td>
<td>Data Coordinator</td>
<td>Singapore</td>
<td>64</td>
<td>2012/04/09</td>
<td>8,575</td>
</tr>
<tr>
<td>Zenaida Frank</td>
<td>Software Engineer</td>
<td>New York</td>
<td>63</td>
<td>2010/01/04</td>
<td>5,250</td>
</tr>
<tr>
<td>Zorita Serrano</td>
<td>Software Engineer</td>
<td>San Francisco</td>
<td>56</td>
<td>2012/06/01</td>
<td>5,000</td>
</tr>
<tr>
<td>Jennifer Acosta</td>
<td>Junior Javascript Developer</td>
<td>Edinburgh</td>
<td>43</td>
<td>2013/02/01</td>
<td>,650</td>
</tr>
<tr>
<td>Cara Stevens</td>
<td>Sales Assistant</td>
<td>New York</td>
<td>46</td>
<td>2011/12/06</td>
<td>5,600</td>
</tr>
<tr>
<td>Hermione Butler</td>
<td>Regional Director</td>
<td>London</td>
<td>47</td>
<td>2011/03/21</td>
<td>6,250</td>
</tr>
<tr>
<td>Lael Greer</td>
<td>Systems Administrator</td>
<td>London</td>
<td>21</td>
<td>2009/02/27</td>
<td>3,500</td>
</tr>
<tr>
<td>Jonas Alexander</td>
<td>Developer</td>
<td>San Francisco</td>
<td>30</td>
<td>2010/07/14</td>
<td>,500</td>
</tr>
<tr>
<td>Shad Decker</td>
<td>Regional Director</td>
<td>Edinburgh</td>
<td>51</td>
<td>2008/11/13</td>
<td>3,000</td>
</tr>
<tr>
<td>Michael Bruce</td>
<td>Javascript Developer</td>
<td>Singapore</td>
<td>29</td>
<td>2011/06/27</td>
<td>3,000</td>
</tr>
<tr>
<td>Donna Snider</td>
<td>Customer Support</td>
<td>New York</td>
<td>27</td>
<td>2011/01/25</td>
<td>2,000</td>
</tr>
</tbody>
</table>
我很快将您的代码发布到 Datatable 网站的一个示例中，如果您向下滚动到 table 下方，您会看到您的长度菜单确实出现了。
（请注意，我也向这个 fiddle 添加了必要的外部文件，对 jQuery 的引用和对缩小的 datatables.js 文件的引用。所以请确保你'在您自己的标记中也引用它们！）
如果您希望将长度菜单重新定位到 table 上方，为了更好的过滤可见性，您可以移动 "l" (lengthMenu)在 "Dom" 到 "t" 之前 (table).
检查 select 元素在 css 文件中是否有 显示：none，改成display:block or inline-block..我就是这样。
您好，我设置了一个长度菜单，我的数据表版本是 DataTables 1.10.13
"dom": '<f<t><"#df"<"pull-left" i><"pull-right"p><"pull-right"l>>>',
"lengthChange": true,
"lengthMenu": [[10, 20, 30, -1], [10, 20, 30, "All"]],
但是没有显示长度菜单，我用火检查，但是发现 .dataTables_length 没有更改为 display: none;。根据我的理解，我认为这个 class 由 "lengthChange": true,，那为什么它不起作用？请帮忙。
.dataTables_length {
display: none;
}
https://jsfiddle.net/4z3ahj70/
JS
$(document).ready(function() {
$('#example').DataTable( {
"dom": '<f<t><"#df"<"pull-left" i><"pull-right"p><"pull-right"l>>>',
"lengthChange": true,
"lengthMenu": [[10, 20, 30, -1], [10, 20, 30, "All"]],
});
});
HTML
<table id="example" class="display" cellspacing="0" width="100%">
<thead>
<tr>
<th>Name</th>
<th>Position</th>
<th>Office</th>
<th>Age</th>
<th>Start date</th>
<th>Salary</th>
</tr>
</thead>
<tfoot>
<tr>
<th>Name</th>
<th>Position</th>
<th>Office</th>
<th>Age</th>
<th>Start date</th>
<th>Salary</th>
</tr>
</tfoot>
<tbody>
<tr>
<td>Tiger Nixon</td>
<td>System Architect</td>
<td>Edinburgh</td>
<td>61</td>
<td>2011/04/25</td>
<td>0,800</td>
</tr>
<tr>
<td>Garrett Winters</td>
<td>Accountant</td>
<td>Tokyo</td>
<td>63</td>
<td>2011/07/25</td>
<td>0,750</td>
</tr>
<tr>
<td>Ashton Cox</td>
<td>Junior Technical Author</td>
<td>San Francisco</td>
<td>66</td>
<td>2009/01/12</td>
<td>,000</td>
</tr>
<tr>
<td>Cedric Kelly</td>
<td>Senior Javascript Developer</td>
<td>Edinburgh</td>
<td>22</td>
<td>2012/03/29</td>
<td>3,060</td>
</tr>
<tr>
<td>Airi Satou</td>
<td>Accountant</td>
<td>Tokyo</td>
<td>33</td>
<td>2008/11/28</td>
<td>2,700</td>
</tr>
<tr>
<td>Brielle Williamson</td>
<td>Integration Specialist</td>
<td>New York</td>
<td>61</td>
<td>2012/12/02</td>
<td>2,000</td>
</tr>
<tr>
<td>Herrod Chandler</td>
<td>Sales Assistant</td>
<td>San Francisco</td>
<td>59</td>
<td>2012/08/06</td>
<td>7,500</td>
</tr>
<tr>
<td>Rhona Davidson</td>
<td>Integration Specialist</td>
<td>Tokyo</td>
<td>55</td>
<td>2010/10/14</td>
<td>7,900</td>
</tr>
<tr>
<td>Colleen Hurst</td>
<td>Javascript Developer</td>
<td>San Francisco</td>
<td>39</td>
<td>2009/09/15</td>
<td>5,500</td>
</tr>
<tr>
<td>Sonya Frost</td>
<td>Software Engineer</td>
<td>Edinburgh</td>
<td>23</td>
<td>2008/12/13</td>
<td>3,600</td>
</tr>
<tr>
<td>Jena Gaines</td>
<td>Office Manager</td>
<td>London</td>
<td>30</td>
<td>2008/12/19</td>
<td>,560</td>
</tr>
<tr>
<td>Quinn Flynn</td>
<td>Support Lead</td>
<td>Edinburgh</td>
<td>22</td>
<td>2013/03/03</td>
<td>2,000</td>
</tr>
<tr>
<td>Charde Marshall</td>
<td>Regional Director</td>
<td>San Francisco</td>
<td>36</td>
<td>2008/10/16</td>
<td>0,600</td>
</tr>
<tr>
<td>Haley Kennedy</td>
<td>Senior Marketing Designer</td>
<td>London</td>
<td>43</td>
<td>2012/12/18</td>
<td>3,500</td>
</tr>
<tr>
<td>Tatyana Fitzpatrick</td>
<td>Regional Director</td>
<td>London</td>
<td>19</td>
<td>2010/03/17</td>
<td>5,750</td>
</tr>
<tr>
<td>Michael Silva</td>
<td>Marketing Designer</td>
<td>London</td>
<td>66</td>
<td>2012/11/27</td>
<td>8,500</td>
</tr>
<tr>
<td>Paul Byrd</td>
<td>Chief Financial Officer (CFO)</td>
<td>New York</td>
<td>64</td>
<td>2010/06/09</td>
<td>5,000</td>
</tr>
<tr>
<td>Gloria Little</td>
<td>Systems Administrator</td>
<td>New York</td>
<td>59</td>
<td>2009/04/10</td>
<td>7,500</td>
</tr>
<tr>
<td>Bradley Greer</td>
<td>Software Engineer</td>
<td>London</td>
<td>41</td>
<td>2012/10/13</td>
<td>2,000</td>
</tr>
<tr>
<td>Dai Rios</td>
<td>Personnel Lead</td>
<td>Edinburgh</td>
<td>35</td>
<td>2012/09/26</td>
<td>7,500</td>
</tr>
<tr>
<td>Jenette Caldwell</td>
<td>Development Lead</td>
<td>New York</td>
<td>30</td>
<td>2011/09/03</td>
<td>5,000</td>
</tr>
<tr>
<td>Yuri Berry</td>
<td>Chief Marketing Officer (CMO)</td>
<td>New York</td>
<td>40</td>
<td>2009/06/25</td>
<td>5,000</td>
</tr>
<tr>
<td>Caesar Vance</td>
<td>Pre-Sales Support</td>
<td>New York</td>
<td>21</td>
<td>2011/12/12</td>
<td>6,450</td>
</tr>
<tr>
<td>Doris Wilder</td>
<td>Sales Assistant</td>
<td>Sidney</td>
<td>23</td>
<td>2010/09/20</td>
<td>,600</td>
</tr>
<tr>
<td>Angelica Ramos</td>
<td>Chief Executive Officer (CEO)</td>
<td>London</td>
<td>47</td>
<td>2009/10/09</td>
<td>,200,000</td>
</tr>
<tr>
<td>Gavin Joyce</td>
<td>Developer</td>
<td>Edinburgh</td>
<td>42</td>
<td>2010/12/22</td>
<td>,575</td>
</tr>
<tr>
<td>Jennifer Chang</td>
<td>Regional Director</td>
<td>Singapore</td>
<td>28</td>
<td>2010/11/14</td>
<td>7,650</td>
</tr>
<tr>
<td>Brenden Wagner</td>
<td>Software Engineer</td>
<td>San Francisco</td>
<td>28</td>
<td>2011/06/07</td>
<td>6,850</td>
</tr>
<tr>
<td>Fiona Green</td>
<td>Chief Operating Officer (COO)</td>
<td>San Francisco</td>
<td>48</td>
<td>2010/03/11</td>
<td>0,000</td>
</tr>
<tr>
<td>Shou Itou</td>
<td>Regional Marketing</td>
<td>Tokyo</td>
<td>20</td>
<td>2011/08/14</td>
<td>3,000</td>
</tr>
<tr>
<td>Michelle House</td>
<td>Integration Specialist</td>
<td>Sidney</td>
<td>37</td>
<td>2011/06/02</td>
<td>,400</td>
</tr>
<tr>
<td>Suki Burks</td>
<td>Developer</td>
<td>London</td>
<td>53</td>
<td>2009/10/22</td>
<td>4,500</td>
</tr>
<tr>
<td>Prescott Bartlett</td>
<td>Technical Author</td>
<td>London</td>
<td>27</td>
<td>2011/05/07</td>
<td>5,000</td>
</tr>
<tr>
<td>Gavin Cortez</td>
<td>Team Leader</td>
<td>San Francisco</td>
<td>22</td>
<td>2008/10/26</td>
<td>5,500</td>
</tr>
<tr>
<td>Martena Mccray</td>
<td>Post-Sales support</td>
<td>Edinburgh</td>
<td>46</td>
<td>2011/03/09</td>
<td>4,050</td>
</tr>
<tr>
<td>Unity Butler</td>
<td>Marketing Designer</td>
<td>San Francisco</td>
<td>47</td>
<td>2009/12/09</td>
<td>,675</td>
</tr>
<tr>
<td>Howard Hatfield</td>
<td>Office Manager</td>
<td>San Francisco</td>
<td>51</td>
<td>2008/12/16</td>
<td>4,500</td>
</tr>
<tr>
<td>Hope Fuentes</td>
<td>Secretary</td>
<td>San Francisco</td>
<td>41</td>
<td>2010/02/12</td>
<td>9,850</td>
</tr>
<tr>
<td>Vivian Harrell</td>
<td>Financial Controller</td>
<td>San Francisco</td>
<td>62</td>
<td>2009/02/14</td>
<td>2,500</td>
</tr>
<tr>
<td>Timothy Mooney</td>
<td>Office Manager</td>
<td>London</td>
<td>37</td>
<td>2008/12/11</td>
<td>6,200</td>
</tr>
<tr>
<td>Jackson Bradshaw</td>
<td>Director</td>
<td>New York</td>
<td>65</td>
<td>2008/09/26</td>
<td>5,750</td>
</tr>
<tr>
<td>Olivia Liang</td>
<td>Support Engineer</td>
<td>Singapore</td>
<td>64</td>
<td>2011/02/03</td>
<td>4,500</td>
</tr>
<tr>
<td>Bruno Nash</td>
<td>Software Engineer</td>
<td>London</td>
<td>38</td>
<td>2011/05/03</td>
<td>3,500</td>
</tr>
<tr>
<td>Sakura Yamamoto</td>
<td>Support Engineer</td>
<td>Tokyo</td>
<td>37</td>
<td>2009/08/19</td>
<td>9,575</td>
</tr>
<tr>
<td>Thor Walton</td>
<td>Developer</td>
<td>New York</td>
<td>61</td>
<td>2013/08/11</td>
<td>,540</td>
</tr>
<tr>
<td>Finn Camacho</td>
<td>Support Engineer</td>
<td>San Francisco</td>
<td>47</td>
<td>2009/07/07</td>
<td>,500</td>
</tr>
<tr>
<td>Serge Baldwin</td>
<td>Data Coordinator</td>
<td>Singapore</td>
<td>64</td>
<td>2012/04/09</td>
<td>8,575</td>
</tr>
<tr>
<td>Zenaida Frank</td>
<td>Software Engineer</td>
<td>New York</td>
<td>63</td>
<td>2010/01/04</td>
<td>5,250</td>
</tr>
<tr>
<td>Zorita Serrano</td>
<td>Software Engineer</td>
<td>San Francisco</td>
<td>56</td>
<td>2012/06/01</td>
<td>5,000</td>
</tr>
<tr>
<td>Jennifer Acosta</td>
<td>Junior Javascript Developer</td>
<td>Edinburgh</td>
<td>43</td>
<td>2013/02/01</td>
<td>,650</td>
</tr>
<tr>
<td>Cara Stevens</td>
<td>Sales Assistant</td>
<td>New York</td>
<td>46</td>
<td>2011/12/06</td>
<td>5,600</td>
</tr>
<tr>
<td>Hermione Butler</td>
<td>Regional Director</td>
<td>London</td>
<td>47</td>
<td>2011/03/21</td>
<td>6,250</td>
</tr>
<tr>
<td>Lael Greer</td>
<td>Systems Administrator</td>
<td>London</td>
<td>21</td>
<td>2009/02/27</td>
<td>3,500</td>
</tr>
<tr>
<td>Jonas Alexander</td>
<td>Developer</td>
<td>San Francisco</td>
<td>30</td>
<td>2010/07/14</td>
<td>,500</td>
</tr>
<tr>
<td>Shad Decker</td>
<td>Regional Director</td>
<td>Edinburgh</td>
<td>51</td>
<td>2008/11/13</td>
<td>3,000</td>
</tr>
<tr>
<td>Michael Bruce</td>
<td>Javascript Developer</td>
<td>Singapore</td>
<td>29</td>
<td>2011/06/27</td>
<td>3,000</td>
</tr>
<tr>
<td>Donna Snider</td>
<td>Customer Support</td>
<td>New York</td>
<td>27</td>
<td>2011/01/25</td>
<td>2,000</td>
</tr>
</tbody>
</table>
我很快将您的代码发布到 Datatable 网站的一个示例中，如果您向下滚动到 table 下方，您会看到您的长度菜单确实出现了。
（请注意，我也向这个 fiddle 添加了必要的外部文件，对 jQuery 的引用和对缩小的 datatables.js 文件的引用。所以请确保你'在您自己的标记中也引用它们！）
如果您希望将长度菜单重新定位到 table 上方，为了更好的过滤可见性，您可以移动 "l" (lengthMenu)在 "Dom" 到 "t" 之前 (table).
检查 select 元素在 css 文件中是否有 显示：none，改成display:block or inline-block..我就是这样。