function

Overview

The function node executes JavaScript code to process messages. The most powerful and flexible node for data transformation, calculations, and custom logic. Access flow/global context, use external libraries, and create complex workflows.

Properties

Available Objects

msg.payload  // Main data
msg.topic    // Message topic
msg._msgid   // Unique ID

node.send(msg)   // Send message
node.warn("...")  // Log warning
node.error("...")  // Log error

context.get("key")
context.set("key", value)

flow.get("key")
global.set("key", value)

Example: Data Transformation

Convert Celsius to Fahrenheit with validation.

// Convert temperature and add metadata
var celsius = msg.payload.temperature;

if (typeof celsius !== 'number' || isNaN(celsius)) {
    node.warn("Invalid temperature value");
    return null; // Drop message
}

msg.payload = {
    celsius: celsius,
    fahrenheit: (celsius * 9/5) + 32,
    kelvin: celsius + 273.15,
    timestamp: Date.now(),
    status: celsius > 30 ? "high" : celsius < 10 ? "low" : "normal"
};

return msg;

Example: Multiple Outputs (Router)

Route messages to different outputs based on conditions.

// Route by severity (3 outputs configured)
var value = msg.payload.value;
var threshold = msg.payload.threshold || 50;

if (value > threshold * 1.5) {
    // Critical - Output 1
    msg.severity = "critical";
    return [msg, null, null];
} else if (value > threshold) {
    // Warning - Output 2
    msg.severity = "warning";
    return [null, msg, null];
} else {
    // Normal - Output 3
    msg.severity = "normal";
    return [null, null, msg];
}

Example: Using Context Storage

Calculate rolling average using flow context.

// Calculate rolling average (last 10 values)
var readings = flow.get("readings") || [];
readings.push(msg.payload);

// Keep only last 10
if (readings.length > 10) {
    readings.shift();
}

flow.set("readings", readings);

// Calculate average
var sum = readings.reduce((a, b) => a + b, 0);
var average = sum / readings.length;

msg.payload = {
    current: msg.payload,
    average: average.toFixed(2),
    samples: readings.length,
    min: Math.min(...readings),
    max: Math.max(...readings)
};

return msg;

Example: Async Operations

Handle asynchronous operations with node.send().

// Async with setTimeout (simulating delay)
setTimeout(function() {
    msg.payload = "Delayed message: " + msg.payload;
    node.send(msg);
}, 1000);

// Important: return nothing for async
return null;

// Alternative: Using promises
// const result = await someAsyncFunction();
// msg.payload = result;
// return msg;

Best Practices