<textarea id="area" style="width:80%;height:60px">
Selecting in this text updates values below.
</textarea>
<br>
From <input id="from" disabled> – To <input id="to" disabled>

<script>
  area.onselect = function() {
    from.value = area.selectionStart;
    to.value = area.selectionEnd;
  };
</script>