document.activeElement

by JM note

HTML

<body id='body'>
  <textarea id='textarea1'>안녕하세요</textarea>
  <textarea id='textarea2'>반갑습니다</textarea>
  <br>선택된 요소:
  <input type='text' id='text1' />
</body>

<script>
  function showSelected(e) {
    document.getElementById("text1").value = document.activeElement.id;
  }
  document.addEventListener("mouseup", showSelected, false);

</script>