Node-RED Cheat Sheet - Function Node

Examples of javascript code which you can use in Node-RED function nodes.

Table of Contents

• Log to debug panel
• Use global variable
• Use flow variable
• Send (multiple) output message
• Show state icon
• New JSON message
• Convert String to Integer
• Send to a dynamic topic
• Convert Boolean to OFF/ON
• Compare two temperatures

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Log to debug panel

node.error("Log line");

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Use global variable

global.set('rain', true); 

global.get('rain'); 

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Use flow variable

flow.set('rain', true); 

flow.get('rain'); 

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Send (multiple) output message

node.send({ payload : true });

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Show state icon

It’s possible add a status icon under the function node and show a internal value next to it.

In this case it shows a green round with value 0.

var diff = 0;
node.status( { fill: "green", shape: "ring", text: diff } );

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New JSON message

Create a new JSON message based on the input JSON message.

var temp = msg.payload;
msg.payload = {
    "volt": temp.voltage,
	"power": temp.power,
	"state": temp.status,
	"device": "smartplug"
}
return msg;

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Convert String to Integer

“20” -> 20

var intValue = parseInt("20");

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Send to a dynamic topic

Define the MQTT topic name in code.

If the incoming variable action='aan' is set, the payload will be set to ON and otherwise it will be OFF.

var deviceId = "bulb1";
msg = {}
msg.payload = (action === "aan") ? "ON" : "OFF";
msg.topic = "zigbee2mqtt/" + deviceId + "/set";
return msg;

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Convert Boolean to OFF/ON

If the input payload contains the field contact and the value is true it will be converted to the text OFF.

This can be used to control a smartplug based on a contact sensor.

{ “contact” : true }		“OFF”

if (msg.payload.contact === true) { 
  msg.payload = "OFF" 
} else {
  msg.payload = "ON";
}
return msg;

You can also write the same functionality with less code:

msg.payload = (msg.payload.contact) ? "OFF" : "ON";
return msg;

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Compare two temperatures

Compare two temperature values and when the difference is less than 3 pass the message otherwise don’t pass.

The first temperature is stored in the flow variable flow.temp, the next temperature comes in as the payload value.

24.1		24.1 null

var temp = flow.get('temp');
var diff = Math.round(temp - msg.payload);
if (diff < 3) {
    // temp diff is acceptable
    node.status({fill:"green",shape:"ring",text:diff})
    return msg;
} else {
    node.status({fill:"red",shape:"ring",text:diff})
    return null;
}

return msg;

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Table of Content ^

<< See also my other Node-RED pages

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