Set up a LoRaWAN payload formatter on The Things Stack to decode uplink bytes
domain: the-things-stack · 6 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Navigate to Applications > your application > Payload Formatters > Uplink in The Things Stack console.
Select 'Custom Javascript formatter' and implement a decodeUplink(input) function that reads input.bytes (a JavaScript byte array), input.fPort, and input.recvTime, and returns an object with a data field containing the decoded measurements.
Return a warnings array in the output object for non-fatal decode issues; return errors for fatal ones — messages with errors are still forwarded but flagged.
Test the formatter using the built-in 'Test' panel by supplying a sample hex payload and FPort before saving.
Keep the formatter script under the documented size limit (currently 40 KB for inline scripts; scripts loaded from the Device Repository can be larger).
For downlink commands, implement an encodeDownlink(input) function that converts input.data fields into a bytes array for the target FPort.
Known gotchas
The formatter runs in a sandboxed JavaScript environment without access to external libraries or network calls; all decoding logic must be self-contained.
Returning an invalid type from decodeUplink (e.g., a non-object or missing the data field) causes the formatter to silently fail and the raw bytes are forwarded instead.
Application-level formatters override device-level formatters set in the device repository; check which scope is active if decoded output is unexpected.
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