textareaで<TAB>入力したい

こういうhtmlがあったとして

<textarea id="tab"></textarea>

こう書くといいかんじ
（DOM取得周りとかaddEventListenerとattachEventの差異は適当に埋めてください）

document.getElementById('tab').addEventListener('keydown', function (e) {
  var elem, end, start, value;
  if (e.keyCode === 9) {
    if (e.preventDefault) {
      e.preventDefault();
    }
    elem = e.target;
    start = elem.selectionStart;
    end = elem.selectionEnd;
    value = elem.value;
    elem.value = "" + (value.substring(0, start)) + "\t" + (value.substring(end));
    elem.selectionStart = elem.selectionEnd = start + 1;
    return false;
  }
});

おまけ: CoffeeScriptだとこう（上記コードの変換元）

document.getElementById('tab').addEventListener 'keydown', (e) ->
  if e.keyCode is 9
    e.preventDefault() if e.preventDefault
    elem = e.target
    start = elem.selectionStart
    end = elem.selectionEnd
    value = elem.value
    elem.value = "#{value.substring 0, start}\t#{value.substring end}"
    elem.selectionStart = elem.selectionEnd = start + 1
    false