JS insert letter into input at the selection

letter_insert.js

insert_letter: function (input_id, letter) {
  var elem = document.getElementById(input_id);
  var selectionStart = elem.selectionStart;
  var selectionEnd = elem.selectionEnd;
  var start = elem.value.substring(0, selectionStart);
  var finish = elem.value.substring(selectionEnd, elem.value.length);
  elem.value = start + letter + finish;
  elem.focus();
  elem.selectionStart = selectionStart + 1;
  elem.selectionEnd = selectionStart + 1;
}