function

Overview

The function node lets you run arbitrary JavaScript code against each incoming message. Write any transformation, calculation, or decision logic that cannot be expressed with built-in nodes. The code receives msg as input and the return value is stored in msg.payload[output_key].

Properties

Execution Features

// Read properties
const temp = msg.payload.temperature;
const topic = msg.topic;

// Modify and return
msg.payload.converted = temp * 9/5 + 32;
return msg;

// outputs: 2
if (msg.payload.value > 100) {
  return [msg, null]; // output 1 only
} else {
  return [null, msg]; // output 2 only
}

// Store a running count
let count = context.get('count') || 0;
count += 1;
context.set('count', count);
msg.payload.count = count;
return msg;

// Filter out low values
if (msg.payload.value < 10) {
  return null; // message dropped
}
return msg;

Example: Temperature Conversion

Convert Celsius readings to Fahrenheit and add a human-readable label.

// Function node configuration
{
  "code": "const c = msg.payload.temperature;\nconst f = c * 9/5 + 32;\nmsg.payload.fahrenheit = Math.round(f * 10) / 10;\nmsg.payload.label = f > 100 ? 'HOT' : f > 60 ? 'WARM' : 'COOL';\nreturn msg;",
  "output_key": "result"
}

// Input message
{
  "topic": "sensors/room",
  "payload": { "temperature": 23.5 }
}

// Output message
{
  "topic": "sensors/room",
  "payload": {
    "temperature": 23.5,
    "fahrenheit": 74.3,
    "label": "WARM"
  }
}

Example: Data Transformation & Filtering

Filter an array of sensor readings and compute aggregate statistics.

// Function node code
{
  "code": "const readings = msg.payload.readings;\nconst valid = readings.filter(r => r.value !== null && r.value > 0);\nconst avg = valid.reduce((sum, r) => sum + r.value, 0) / valid.length;\nconst max = Math.max(...valid.map(r => r.value));\nconst min = Math.min(...valid.map(r => r.value));\nmsg.payload = { avg: Math.round(avg*100)/100, max, min, count: valid.length };\nreturn msg;"
}

// Input: 10 sensor readings (some null)
{
  "payload": {
    "readings": [
      { "id": 1, "value": 22.5 },
      { "id": 2, "value": null },
      { "id": 3, "value": 24.1 },
      { "id": 4, "value": 23.8 }
    ]
  }
}

// Output: aggregated stats
{
  "payload": { "avg": 23.47, "max": 24.1, "min": 22.5, "count": 3 }
}

Common Use Cases