const tx = document.getElementsByTagName("textarea");
for (let i = 0; i < tx.length; i++) {
  tx[i].setAttribute(
    "style",
    tx[i].getAttribute("style") +
      ";height:" +
      (tx[i].scrollHeight + 4) +
      "px;overflow-y:hidden;"
  );
  tx[i].addEventListener("change", resize_textarea, false);
}

function resize_textarea(t) {
  t = t instanceof Element ? t : this;
  t.style.height = "auto";
  t.style.height = t.scrollHeight + 4 + "px";
}