code: function utf8FillLast(buf) {
    var p = this.lastTotal - this.lastNeed;
    var r = utf8CheckExtraBytes(this, buf, p);
    if (r !== undefined) return r;
    if (this.lastNeed <= buf.length) {
      buf.copy(this.lastChar, p, 0, this.lastNeed);
      return this.lastChar.toString(this.encoding, 0, this.lastTotal);
    }
    buf.copy(this.lastChar, p, 0, buf.length);
    this.lastNeed -= buf.length;
  }
docs: Attempts to complete a multi-byte UTF-8 character using bytes from a Buffer.