Make a TTN v2 payload decoder work with TTI/N v3

decoder.js

function Decoder(bytes, port) {
  // Your "old" TTN v2 decoder function
}

// add other functions (Converter, Validator) here if you used them. If you didn't you can just skip them.

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

  if (typeof Decoder === "function") {
    data = Decoder(data, input.fPort);
  }

  if (typeof Converter === "function") {
    data = Converter(data, input.fPort);
  }

  if (typeof Validator === "function") {
    valid = Validator(data, input.fPort);
  }

  if (valid) {
    return {
      data: data
    };
  } else {
    return {
      data: {},
      errors: ["Invalid data received"]
    };
  }
}

You need to return the decoded object, and then they will be. I see where you might be coming from. If you use the following Decoder function you should get what you're aiming for:

function Decoder(bytes, port) {
  // Decode an uplink message from a buffer
  // (array) of bytes to an object of fields.
  var decoded = {};
  
  if (port === 2) decoded.b = bytes[0]/50;
  if (port === 2) decoded.at = bytes[1]-100;

  return decoded;
}