Untitled


<html>
	<head>
		<meta charset="utf-8" />
	</head>

	<form name="selectForm">
  <p>
    <label for="musicTypes">Wybierz ulubione gatunki muzyki: </label>
    <select id="musicTypes" name="musicTypes" multiple="multiple">
      <option selected="selected">Rege</option>
      <option>Jazz</option>
      <option>Blues</option>
      <option>New Age</option>
      <option>Classical</option>
      <option>Opera</option>
    </select>
  </p>
  <p><input id="btn" type="button" value="Ile gatunków wybrano?" /></p>
</form>

<script>
function howMany(selectObject) {
  var numberSelected = 0;
  for (var i = 0; i < selectObject.options.length; i++) {
    if (selectObject.options[i].selected) {
      numberSelected++;
    }
  }
  return numberSelected;
}

var btn = document.getElementById("btn");
btn.addEventListener("click", function(){
  alert('Ilość zaznaczonych obiektów ' + howMany(document.selectForm.musicTypes))
});
</script>
	</body>
</html>