Table HTML manipulée par Javascript

:salut:

Le message d'erreur vous dit que vous essayez de manipuler des cellules inexistantes (débordement d'index).

Voici un exemple, avec un code de "table" correct.
Code:
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<!DOCTYPE html>
<html lang="fr" dir="ltr">
<head>
  <meta http-equiv="cache-control" content="public, max-age=60">
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <meta name="author" content="Daniel Hagnoul">
  <title>Test</title>
  <style>
    
  </style>
  <script>
    'use strict';
    
    document.addEventListener( 'DOMContentLoaded', ev => {
      
    }, false );
    
    window.addEventListener( 'load', ev => {
      
      document.querySelector( "#btnRemove" ).addEventListener( 'click', ev => {
        const
          tbody = document.querySelector( "#tab > tbody" );
          
        let
          row,
          cells;
          
        for( let n = 0; n < tbody.rows.length; n++ ){
          row = tbody.rows[ n ];
          cells = row.cells;
          
          if ( ( n + 1  < tbody.rows.length ) && ( n % 2 === 0 ) ){
            
            // rowSpan 2 pour cell 1
            tbody.rows[ n + 1 ].deleteCell( 1 );
            cells[ 1 ].rowSpan = 2;
            
            // rowSpan 2 pour cell 3
            tbody.rows[ n + 1 ].deleteCell( 3 );
            cells[ 3 ].rowSpan = 2;
          }
        }
      }, false );
      
    }, false );
  </script>
</head>
<body>
  <main>
 
    <table border="1" id="tab" style="background-color:#f0f7f6">
      <colgroup>
        <col width="210"/>
        <col width="300"/>
        <col width="300"/>
        <col width="300"/>
        <col width="300"/>
        <col width="300"/>
      </colgroup>
      <thead>
        <tr>
          <!-- if checkbox is checked, clone school subjects to the whole table row  -->
          <th></th>
          <th  style="color:#444;background-color:#e0e0e0;text-align:center">lundi</th>
          <th  style="color:#444;background-color:#e0e0e0;text-align:center">mardi</th>
          <th  style="color:#444;background-color:#e0e0e0;text-align:center">mercredi</th>
          <th  style="color:#444;background-color:#e0e0e0;text-align:center">jeudi</th>
          <th  style="color:#444;background-color:#e0e0e0;text-align:center">vendredi</th>
        </tr>
      </thead>
      <tbody>
        <tr>
          <td valign="top" id="td1" style="color:#444;background-color:#e0e0e0;height:15px;font-size:12px;font-weight:bold;text-align:center">8:00</td>
          <td valign="top" ></td>
          <td valign="top" ></td>
          <td valign="top" ></td>
          <td valign="top" ></td>
          <td valign="top" ></td>
        </tr>
        <tr>
          <td valign="top" id="td2" style="color:#444;background-color:#e0e0e0;height:15px;font-size:12px;font-weight:bold;text-align:center">9:00</td>
          <td valign="top" ></td>
          <td valign="top" ></td>
          <td valign="top" ></td>
          <td valign="top" ></td>
          <td valign="top" ></td>
        </tr>
        <tr>
          <td valign="top" id="td3" style="color:#444;background-color:#e0e0e0;height:15px;font-size:12px;font-weight:bold;text-align:center">9:30</td>
          <td valign="top" ></td>
          <td valign="top" ></td>
          <td valign="top" ></td>
          <td valign="top" ></td>
          <td valign="top" ></td>
        </tr>
        <tr>
          <td valign="top" id="td4" style="color:#444;background-color:#e0e0e0;height:15px;font-size:12px;font-weight:bold;text-align:center">11:00</td>
          <td valign="top" ></td>
          <td valign="top" ></td>
          <td valign="top" ></td>
          <td valign="top" ></td>
          <td valign="top" ></td>
        </tr>
      </tbody>
    </table>
    <br>
    <button id="btnRemove">Remove Cell 24</button>      
 
  </main>
</body>
</html>