テキスト入力フィールドのカーソル位置を取得するにはどうすればよいですか?

/*
Returns the caret (cursor) position of the specified text field (oField).
Return value range is 0-oField.value.length.
*/
function doGetCaretPosition(oField) {
  // Initialize
  var iCaretPos = 0;

  // IE Support
  if (document.selection) {
    // Set focus on the element
    oField.focus();

    // To get cursor position, get empty selection range
    var oSel = document.selection.createRange();

    // Move selection start to 0 position
    oSel.moveStart('character', -oField.value.length);

    // The caret position is selection length
    iCaretPos = oSel.text.length;
  }

  // Firefox support
  else if (oField.selectionStart || oField.selectionStart == '0')
    iCaretPos = oField.selectionDirection=='backward' ? oField.selectionStart : oField.selectionEnd;

  // Return results
  return iCaretPos;
}