I am trying to translate a function from GO to javascript (I barely have any knowledge of GO).

This is the original function

func prepareMessage(data []byte) []byte {
    // Compute CRC before modifying the message.
    crc := crcCompute(data)
    // Add the two CRC16 bytes before replacing control characters.
    data = append(data, byte(crc&0xFF))
    data = append(data, byte(crc>>8))

    tmp := []byte{0x7E} // Flag start.

    // Copy the message over, escaping 0x7E (Flag Byte) and 0x7D (Control-Escape).
    for i := 0; i < len(data); i++ {
        mv := data[i]
        if (mv == 0x7E) || (mv == 0x7D) {
            mv = mv ^ 0x20
            tmp = append(tmp, 0x7D)
        }
        tmp = append(tmp, mv)
    }

    tmp = append(tmp, 0x7E) // Flag end.

    return tmp
}

This is my attempt in JavaScript

 var _appendBuffer = function (buffer1, buffer2) {
    var tmp = new Uint8Array(buffer1.byteLength + buffer2.byteLength);
    tmp.set(new Uint8Array(buffer1), 0);
    tmp.set(new Uint8Array(buffer2), buffer1.byteLength);
    return tmp.buffer;
  };

 function prepareMessage(data) {
    // Compute CRC before modifying the message.
    var crc = crcCompute(data)
    // Add the two CRC16 bytes before replacing control characters.
    data = data + (crc & 0xFF);
    data = data + (crc >> 8);

    var tmp = new ArrayBuffer(1000000000);
    tmp[0] = [0x7E]; // Flag start.

    // Copy the message over, escaping 0x7E (Flag Byte) and 0x7D (Control-Escape).
    for (var i = 0; i < data.length; i++) {
      var mv = data[i]
      if ((mv == 0x7E) || (mv == 0x7D)) {
        mv = mv ^ 0x20;
        tmp = _appendBuffer(tmp, [0x7D]);
      }
      tmp = _appendBuffer(tmp, mv);
    }

    tmp = _appendBuffer(tmp, [0x7E]); // Flag end.
    return tmp;
  }

Getting this error in the prepareMessage function RangeError: Range consisting of offset and length are of bounds at line tmp = _appendBuffer(tmp, mv);

I understand that something's wrong with the buffer sizes, but I don't get how to fix it.

Thanks Leon