HTMLTextAreaElement

function autoGrow(oField) {
  if (oField.scrollHeight > oField.clientHeight) {
    oField.style.height = `${oField.scrollHeight}px`;
  }
}

textarea.noscrollbars {
  overflow: hidden;
  width: 300px;
  height: 100px;
}

<form>
  <fieldset>
    <legend>あなたのコメント</legend>
    <p><textarea class="noscrollbars" onkeyup="autoGrow(this);"></textarea></p>
    <p><input type="submit" value="送信" /></p>
  </fieldset>
</form>

function insert(startTag, endTag) {
  const textArea = document.myForm.myTxtArea;
  const selectionStart = textArea.selectionStart;
  const selectionEnd = textArea.selectionEnd;
  const oldText = textArea.value;

  const prefix = oldText.substring(0, selectionStart);
  const inserted =
    startTag + oldText.substring(selectionStart, selectionEnd) + endTag;
  const suffix = oldText.substring(selectionEnd);
  textArea.value = `${prefix}${inserted}${suffix}`;

  const newSelectionStart = selectionStart + startTag.length;
  const newSelectionEnd = selectionEnd + startTag.length;
  textArea.setSelectionRange(newSelectionStart, newSelectionEnd);

  textArea.focus();
}

function insertURL() {
  const newURL = prompt("リンクの完全な URL を入力してください");
  if (newURL) {
    insert(`<a href="${newURL}">`, "</a>");
  } else {
    document.myForm.myTxtArea.focus();
  }
}

const strong = document.querySelector("#format-strong");
const em = document.querySelector("#format-em");
const link = document.querySelector("#format-link");
const code = document.querySelector("#format-code");

strong.addEventListener("click", (e) => insert("<strong>", "</strong>"));
em.addEventListener("click", (e) => insert("<em>", "</em>"));
link.addEventListener("click", (e) => insertURL());
code.addEventListener("click", (e) => insert("\n<code>\n", "\n</code>\n"));

.intLink {
  cursor: pointer;
  text-decoration: underline;
  color: #0000ff;
}

<form name="myForm">
  <p>
    [&nbsp;
    <span class="intLink" id="format-strong"><strong>Bold</strong></span> |
    <span class="intLink" id="format-em"><em>Italic</em></span> |
    <span class="intLink" id="format-link">URL</span> |
    <span class="intLink" id="format-code">code</span> &nbsp;]
  </p>

  <p>
    <textarea name="myTxtArea" rows="10" cols="50">
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut facilisis, arcu vitae adipiscing placerat, nisl lectus accumsan nisi, vitae iaculis sem neque vel lectus. Praesent tristique commodo lorem quis fringilla. Sed ac tellus eros. Sed consectetur eleifend felis vitae luctus. Praesent sagittis, est eget bibendum tincidunt, ligula diam tincidunt augue, a fermentum odio velit eget mi. Phasellus mattis, elit id fringilla semper, orci magna cursus ligula, non venenatis lacus augue sit amet dui. Pellentesque lacinia odio id nisi pulvinar commodo tempus at odio. Ut consectetur eros porttitor nunc mollis ultrices. Aenean porttitor, purus sollicitudin viverra auctor, neque erat blandit sapien, sit amet tincidunt massa mi ac nibh. Proin nibh sem, bibendum ut placerat nec, cursus et lacus. Phasellus vel augue turpis. Nunc eu mauris eu leo blandit mollis interdum eget lorem.
    </textarea>
  </p>
</form>

function checkRows(oField, oKeyEvent) {
  let nKey = (
      oKeyEvent ||
      /* old IE */ window.event || /* check is not supported! */ { keyCode: 38 }
    ).keyCode,
    // 行の最大文字数をここに入力
    nCols = 30,
    // 最大行数をここに入力
    nRows = 5,
    nSelS = oField.selectionStart,
    nSelE = oField.selectionEnd,
    sVal = oField.value,
    nLen = sVal.length,
    nBackward = nSelS >= nCols ? nSelS - nCols : 0,
    nDeltaForw =
      sVal
        .substring(nBackward, nSelS)
        .search(new RegExp(`\\n(?!.{0,${String(nCols - 2)}}\\n)`)) + 1,
    nRowStart = nBackward + nDeltaForw,
    aReturns = (
      sVal.substring(0, nSelS) + sVal.substring(nSelE, sVal.length)
    ).match(/\n/g),
    nRowEnd = nSelE + nRowStart + nCols - nSelS,
    sRow =
      sVal.substring(nRowStart, nSelS) +
      sVal.substring(nSelE, nRowEnd > nLen ? nLen : nRowEnd),
    bKeepCols =
      nKey === 13 ||
      nLen + 1 < nCols ||
      /\n/.test(sRow) ||
      ((nRowStart === 0 || nDeltaForw > 0 || nKey > 0) &&
        (sRow.length < nCols ||
          (nKey > 0 && (nLen === nRowEnd || sVal.charAt(nRowEnd) === "\n"))));

  return (
    (nKey !== 13 || (aReturns ? aReturns.length + 1 : 1) < nRows) &&
    ((nKey > 32 && nKey < 41) || bKeepCols)
  );
}

<form>
  <p>
    1行の文字数が固定されたテキストエリア:<br />
    <textarea
      cols="50"
      rows="10"
      onkeypress="return checkRows(this, event);"
      onpaste="return false;"></textarea>
  </p>
</form>