function getSel() {
  var editdoc = document.getElementById('tt');
  if (typeof editdoc.selectionStart != 'undefined') {
    return editdoc.value.substr(editdoc.selectionStart, editdoc.selectionEnd - editdoc.selectionStart);
  } else if(document.selection && document.selection.createRange) {
    return document.selection.createRange().text;
  } else {
    return false;
  }
}
$('#tt').mouseup(function() {
  alert(getSel());
});