title: Disable button when no option is selected from dropdown
body: <p>I have an input type button (it can not be submit type because it triggers a secondary action inside a form) that I need to maintain disabled if no selection made on a previous dropdown selector. I have unsuccessfuly tried: <a href="https://stackoverflow.com/questions/31911957/how-to-disable-button-if-i-select-no-in-dropdown-list">this</a>, <a href="https://stackoverflow.com/questions/42207125/disable-button-from-option-select">this</a>, <a href="https://stackoverflow.com/questions/35102464/disable-button-if-no-selected-value-in-dropdown">this</a>, and some others...</p>
  
  <p>here it is my form code :</p>
  
  <pre><code>echo "&lt;select class='corpusname' id='corpusname' size='1' name='corpusname' required /&gt;
  &lt;option value=''&gt;Select a corpus&lt;/option&gt;";
  
  // This query gives the other options from a database
  $result = mysqli_query($db, "SELECT * FROM corpus_info") or die(mysqli_error($db));
  while($cpsmlg = mysqli_fetch_array($result)){
  echo "&lt;option value='".$cpsmlg['corpus']."'&gt;".$cpsmlg['title']."&lt;/option&gt;";
  }
  echo "&lt;/select&gt;
  &lt;a id='theLink' target='_blank'&gt;
  
  // This is the button to be disabled
  &lt;input type='button' id='seedoc' class='seedoc' value='See doc' /&gt;&lt;/a&gt;";
  </code></pre>