Understanding and Using The Node-Red Message Object

A node red flow consists of a series of interconnected nodes.(wired nodes).

All nodes must have an input and can have 0 or multiple outputs.

Nodes exchange data between each other using the msg object.

Each node receives the message object from the previous node, and can then pass this message object onto the next node in the flow.

The Msg Object

The Msg Object is a standard JavaScript object and has several existing properties or parts depending on where it originated.

You can see the message properties by sending the msg to the debug node.

By default the debug node will display the msg.payload property, but you can edit the debug node to display all of the message properties (complete message object).

If you Look at the message object properties of the the inject node.

You will see that is has the following properties or parts

• payload
• topic
• _msgid

A msg object that originates from an MQTT input node has the following properties:

• payload
• topic
• qos
• _msgid
• retain

as shown in the screen shot below:

 

The _msgid property is a message identifier added by node-red and can be used to track message objects.

Msg object properties can be any valid JavaScript type e.g.

• String
• Integer
• Object
• Boolean

Modifying the msg Object

Node-Red provides various core nodes that can change the messages object without you having to write any JavaScript code.

The main nodes are change, split, join, switch

You can find out more of what they do by dragging them onto the flow and then viewing the info tab associated with them.

See the documentation for more details

There is also the very versatile function node, but using it requires writing JavaScript code.

Accessing the msg Properties

You access the msg properties just like you do any JavaScript object.

The message payload can be accessed using:

var payload=msg.payload;

and can be modified using:

msg.payload=payload;

Likewise the message topic can be accessed using:

var topic=msg.topic;

and can be modified using:

msg.topic= topic;

If you are using a node like the change node then it defaults to payload, but you can change any property by editing the appropriate field as shown below:

Adding Properties to the msg Object

You can add properties to the message object. For example if you had an object containing sensor values that looked like this:

sensors={sensor1:20,sensor2:21}

You can add it to msg object using

msg.sensors=sensors

It will be passed on to the next node along with the existing msg properties.

Common Errors

When working with the message object you will probably encounter some of the following errors:

• function tried to send a message of type number node red
• function tried to send a message of type string node red

Both are caused by assigning a string or number to the message object instead of assigning an object e.g

msg=1;
msg="This is invalid";

Another common mistake is to delete the existing msg properties or not forward them on to the next node.

msg={};
return msg;

The following is valid but all the properties in the original msg object are lost to the following node.

msg1={};
msg1.payload="test";
return msg1;

Msg Object and The Node-Red Function Node

Using the function node gives you a better understanding of the node-red msg object see –Using the Node-Red Function Node- Beginners Guide

Video

An Introduction to the Node-Red msg Object

<=====How to Export and Import Nodes and Flows

=====>How to Create a Basic Node-Red Dashboard

Resources:

• Github- Node-red Msg Conventions-Reserved Message properties
• Node Red User Guide –Messages
• Node Red User Guide- writing functions

Related Tutorials:

• Storing Data in Node-Red Variables
• Message and Object Cloning
• Working with JSON data in Node-red
• Understanding and Using Buffers In Node and Node-Red