JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html>


  <style>
    table,
    th,
    td {
      border: 1px solid black;
      border-collapse: collapse;
    }

    th,
    td {
      padding: 5px;
      text-align: left;
    }

  </style>
  </head>


  <div>
    <input type="button" onClick="insertData()" value="Add">
  </div>


  <div class="custom-select" style="width:200px;">
    <select>
      <option value="0">Select operation:</option>
      <option value="1">RUT240</option>
      <option value="2">FMB920</option>
    </select>
  </div>

  <br>



  <table id="t1">
    <caption>Operation table</caption>
    <thead>
      <tr>
        <th>Operation code</th>
        <th>To Do</th>
        <th>Done</th>
        <th>Left</th>
      </tr>
    </thead>
    <tbody>
      <tr>
        <td>RUT240</td>
        <td>1000</td>
        <td>
          <script>
          	get_number_to_pick();
          </script>
        </td>
        <td>13</td>
      </tr>
      <tr>
        <td>FMB920</td>
        <td>555</td>
        <td>
          <script>
          	get_number_to_pick();
          </script>
        </td>
        <td>15</td>
      </tr>
    </tbody>
  </table>

</html>

JavaScript

function get_number_to_pick() {
  var number_to_pick = 50;
  return number_to_pick;
}