join

Overview

The join node collects a sequence of messages and combines them into a single output message. It can build arrays, key-value objects, or concatenated strings. In auto mode it pairs with a preceding split node, while manual and reduce modes offer full control over how messages are accumulated.

Properties

Join Modes

// split → process → join
// Automatically reassembles the
// original array/object/string
// using msg.parts metadata

// count: 5, build: "array"
// Collects 5 messages
// Outputs: [val1, val2, ... val5]

// msg.complete = true
// Forces early output

// Like Array.reduce()
// Initial: 0
// Expression: $A + payload
// Accumulates sum across messages

// key: "topic", count: 3
// msg1: topic="temp", payload=23
// msg2: topic="hum",  payload=65
// msg3: topic="pres", payload=1013
// → { temp:23, hum:65, pres:1013 }

Example: Collect and Average Sensor Readings

Collect 10 temperature readings into an array, then compute the average in a downstream function node.

// Join node configuration
{
  "mode": "manual",
  "build": "array",
  "count": 10,
  "timeout": 60
}

// 10 incoming messages over time:
// msg1.payload = 23.1
// msg2.payload = 23.4
// ...
// msg10.payload = 24.0

// Output (single message):
{
  "payload": [23.1, 23.4, 23.0, 22.8, 23.5, 24.1, 23.9, 24.5, 24.2, 24.0]
}

// Downstream function node:
// const avg = msg.payload.reduce((a,b) => a+b) / msg.payload.length;
// msg.payload = { "average": avg, "samples": msg.payload.length };

Example: Build JSON Object from Multiple Sources

Combine readings from three different sensor topics into a single JSON object for database storage.

// Join node configuration
{
  "mode": "manual",
  "build": "object",
  "count": 3,
  "key": "topic",
  "timeout": 10
}

// Three messages from different MQTT topics:
// msg1: topic="temperature", payload=23.5
// msg2: topic="humidity",    payload=65
// msg3: topic="pressure",    payload=1013.25

// Output (single combined message):
{
  "payload": {
    "temperature": 23.5,
    "humidity": 65,
    "pressure": 1013.25
  }
}

// If timeout (10s) expires before all 3 arrive,
// partial object is sent with available keys

Example: Concatenate Log Lines

Collect log entries and join them into a single string for batch upload.

// Join node configuration
{
  "mode": "manual",
  "build": "string",
  "count": 5,
  "joiner": "\n"
}

// 5 log messages arrive:
// msg1.payload = "[INFO] System started"
// msg2.payload = "[INFO] Connected to broker"
// msg3.payload = "[WARN] High memory usage"
// msg4.payload = "[INFO] Flow deployed"
// msg5.payload = "[INFO] Ready"

// Output:
{
  "payload": "[INFO] System started\n[INFO] Connected to broker\n[WARN] High memory usage\n[INFO] Flow deployed\n[INFO] Ready"
}

Common Use Cases