<textarea id="mytextarea">asdasd asdfsdf asdfsdf dsafsdf</textarea>
<button onclick="getSel()">Click</button>
<script type="text/javascript">
function getSel() // javascript
{
    var txtarea = document.getElementById("mytextarea");
    var start = txtarea.selectionStart;
    var finish = txtarea.selectionEnd;
    var sel = txtarea.value.substring(start, finish);
    console.log(sel);
}
</script>