# Node-RED Function Node Patterns (EVOLV Summary)

Based on: https://nodered.org/docs/user-guide/writing-functions

## Return Semantics

- `return msg;` sends to output 1.
- `return [msg1, msg2, ...];` sends one message per output position.
- Use `null` for outputs that should not emit.

Examples:

- `return [msg, null, null];` -> output 1 only
- `return [null, msg, null];` -> output 2 only

## Message Mutation Pattern

- Start with the incoming `msg`, set fields (`msg.topic`, `msg.payload`), then return/send.
- This preserves message context unless there is a deliberate reason to create a new object.

## Async Function Pattern

- For asynchronous work, call `node.send(...)` and then `node.done()`.
- Avoid returning a message when output is sent asynchronously.

## EVOLV Practical Rule

- In example flows (including parsers), always align return-array position with downstream wiring.
- For dashboard parser functions, keep one stable output mapping per metric to avoid wire-level regressions.