function _isValidUTF8(buf) {
  const len = buf.length;
  let i = 0;

  while (i < len) {
    if ((buf[i] & 0x80) === 0) {
      // 0xxxxxxx
      i++;
    } else if ((buf[i] & 0xe0) === 0xc0) {
      // 110xxxxx 10xxxxxx
      if (
        i + 1 === len ||
        (buf[i + 1] & 0xc0) !== 0x80 ||
        (buf[i] & 0xfe) === 0xc0 // Overlong
      ) {
        return false;
      }

      i += 2;
    } else if ((buf[i] & 0xf0) === 0xe0) {
      // 1110xxxx 10xxxxxx 10xxxxxx
      if (
        i + 2 >= len ||
        (buf[i + 1] & 0xc0) !== 0x80 ||
        (buf[i + 2] & 0xc0) !== 0x80 ||
        (buf[i] === 0xe0 && (buf[i + 1] & 0xe0) === 0x80) || // Overlong
        (buf[i] === 0xed && (buf[i + 1] & 0xe0) === 0xa0) // Surrogate (U+D800 - U+DFFF)
      ) {
        return false;
      }

      i += 3;
    } else if ((buf[i] & 0xf8) === 0xf0) {
      // 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
      if (
        i + 3 >= len ||
        (buf[i + 1] & 0xc0) !== 0x80 ||
        (buf[i + 2] & 0xc0) !== 0x80 ||
        (buf[i + 3] & 0xc0) !== 0x80 ||
        (buf[i] === 0xf0 && (buf[i + 1] & 0xf0) === 0x80) || // Overlong
        (buf[i] === 0xf4 && buf[i + 1] > 0x8f) ||
        buf[i] > 0xf4 // > U+10FFFF
      ) {
        return false;
      }

      i += 4;
    } else {
      return false;
    }
  }

  return true;
}
Checks if a given buffer contains only correct UTF-8.
Ported from https://www.cl.cam.ac.uk/%7Emgk25/ucs/utf8_check.c by
Markus Kuhn.

@param {Buffer} buf The buffer to check
@return {Boolean} `true` if `buf` contains only correct UTF-8, else `false`
@public