Part of the OpendataHN LoRa Feinstaub sensor (https://github.com/opendata-heilbronn/Lora/blob/master/LoRa_Feinstaub_SDS011)

TTN Feinstaub Decoder.js

function Decoder(bytes, port) {
  // Decode an uplink message from a buffer
  // (array) of bytes to an object of fields.
  var decoded = {};
  
  //port 1 = data struct LSB first
  //port 2 = data struct MSB first
  
  if (port === 1) {
    decoded.pm10 = (bytes[1] << 8) + bytes[0];
    decoded.pm25 = (bytes[3] << 8) + bytes[2];
    decoded.temperature = bytesToInt(bytes[5], bytes[4])/100;
    decoded.humidity =    bytesToInt(bytes[7], bytes[6])/100;
    decoded.pressure =    bytesToInt(bytes[9], bytes[8]);
  } else if (port === 2) {
    decoded.pm10 = (bytes[0] << 8) + bytes[1];
    decoded.pm25 = (bytes[2] << 8) + bytes[3];
    decoded.temperature = bytesToInt(bytes[4], bytes[5])/100;
    decoded.humidity =    bytesToInt(bytes[6], bytes[7])/100;
    decoded.pressure =    bytesToInt(bytes[8], bytes[9]);
  }
  
  Object.keys(decoded).forEach(function(key) {
    if(decoded[key] === -300 || decoded[key] === -30000) { //invalid value
      delete decoded[key]
    }
  });
  
  return decoded;
}

function bytesToInt(msb, lsb) {
  if(msb < 128) {
    return (msb << 8) + lsb;
  } else {
    return -((1 << 16) - ((msb << 8) + lsb));
  }
}