ChirpStack Payload Decoder: From Hex to Clean JSON

ChirpStack runs a small JavaScript function on every uplink. It receives the raw bytes and returns a JSON object. The decoded values are then available on the event stream (MQTT/HTTP) and can be forwarded to ThingsBoard, Grafana or any application. The same codec file also holds the downlink encoder for devices you control (relays, thermostats, valves).

ChirpStack v4 expects a decodeUplink(input) function. input.bytes is an array of byte values; you return { data: { ... } }. Milesight devices use a channel-based format, repeated [channel id][channel type][payload]:

function decodeUplink(input) {
  var bytes = input.bytes;
  var data = {};

  for (var i = 0; i < bytes.length; ) {
    var channel = bytes[i++];
    var type = bytes[i++];

    if (channel === 0x01 && type === 0x75) {          // battery (%)
      data.battery = bytes[i]; i += 1;
    } else if (channel === 0x03 && type === 0x67) {   // temperature, INT16 LE / 10
      data.temperature = readInt16LE(bytes, i) / 10; i += 2;
    } else if (channel === 0x04 && type === 0x68) {   // humidity, UINT8 / 2
      data.humidity = bytes[i] / 2; i += 1;
    } else {
      break;  // unknown channel, stop instead of mis-parsing
    }
  }
  return { data: data };
}

function readInt16LE(b, i) {
  var v = (b[i + 1] << 8) | b[i];
  return v > 0x7fff ? v - 0x10000 : v;
}

• Endianness: Milesight payloads are little-endian. Reading multi-byte values in the wrong order gives nonsense numbers.
• Scaling factors: Temperature is divided by 10, humidity by 2. A missing divisor is the most common decode bug.
• Unknown channels: Break on an unrecognised channel instead of mis-parsing the rest of the payload.
• v3 to v4: v3 uses a different function signature and exposed decoded data as objectJSON. v4 exposes it directly as object. Migrate codecs and converters together.