Untitled

<!DOCTYPE html>
<html>

<head>
  <script src="https://code.jquery.com/jquery-3.1.1.min.js" integrity="sha256-hVVnYaiADRTO2PzUGmuLJr8BLUSjGIZsDYGmIJLv2b8=" crossorigin="anonymous"></script>
</head>

<body>
  <li class="als-item">


    <!-- somefunc1 prints undefined -->
    <button onclick="somefunc1(this)" id="mybutton" data-loc-subject="test value">Returns undefined</button>


    <!-- somefunc2 prints the data stored in loc-subject -->
    <button onclick="somefunc2()" id="mybutton" data-loc-subject="test value">Gets the data</button>
    
  </li>

  <script>
    function somefunc1(button) {
      alert($(this).data('loc-subject'));
    }


    function somefunc2() {
      alert($('#mybutton').data('loc-subject'));
    }
  </script>
</body>

</html>