The use of the functions were examples and provide different results. You haven't added the results to the msg.
If you are after both unknown devices and bad signals then try
let msg.unknownDevices = unknownDevices
to get an array of unknownDevices, and after
const result = deviceArray.filter(device => device.signal < minSignal || device.signal > maxSignal)`
use
let msg.badSignal = result
return msg
You will then have to monitor msg.unknownDevices and msg.badSignal instead of msg.payload. Use a debug node set to 'complete message object'
If you just want devices with a bad signal just use
let msg.payload = deviceArray.filter(device => device.signal < minSignal || device.signal > maxSignal)
Note: change minSignal & maxSignal values to whatever values you are actually defining as bad
