JavaScript file

The node .js file defines the runtime behavior of the node.

Node constructor

Nodes are defined by a constructor function that can be used to create new instances of the node. The function gets registered with the runtime so it can be called when nodes of the corresponding type are deployed in a flow.

The function is passed an object containing the properties set in the flow editor.

The first thing it must do is call the RED.nodes.createNode function to initialize the features shared by all nodes. After that, the node-specific code lives.

function SampleNode(config) {
    // node-specific code goes here



Receiving messages

Nodes register a listener on the input event to receive messages from the up-stream nodes in a flow.

With Node-RED 1.0, a new style of the listener was introduced to allow the node to notify the runtime when it has finished handling a message. This added two new parameters to the listener function. Some care is needed to ensure the node can still be installed in Node-RED 0.x which does not use this new style of the listener.

this.on('input', function(msg, send, done) {
    // do something with 'msg'

    // Once finished, call 'done'.
    // This call is wrapped in a check that 'done' exists
    // so the node will work in earlier versions of Node-RED (<1.0)
    if (done) {

Handling errors

If the node encounters an error whilst handling the message, it should pass the details of the error to the done function.

This will trigger any Catch nodes present on the same tab, allowing the user to build flows to handle the error.

Again, some care is needed in the case the node is installed in Node-RED 0.x which does not provide the done function. In that case, it should use node.error:

let node = this;
this.on('input', function(msg, send, done) {
    // do something with 'msg'

    // If an error is hit, report it to the runtime
    if (err) {
        if (done) {
            // Node-RED 1.0 compatible
        } else {
            // Node-RED 0.x compatible
            node.error(err, msg);

Sending messages

If the node sits at the start of the flow and produces messages in response to external events, it should use the send function on the Node object:

var msg = { payload:"hi" }

If the node wants to send from inside the input event listener, in response to receiving a message, it should use the send function that is passed to the listener function:

let node = this;
this.on('input', function(msg, send, done) {
    // For maximum backwards compatibility, check that send exists.
    // If this node is installed in Node-RED 0.x, it will need to
    // fallback to using `node.send`
    send = send || function() { node.send.apply(node,arguments) }

    msg.payload = "hi";

    if (done) {

If msg is null, no message is sent.

If the node is sending a message in response to having received one, it should reuse the received message rather than create a new message object. This ensures existing properties on the message are preserved for the rest of the flow.

Multiple outputs

If the node has more than one output, an array of messages can be passed to send, with each one being sent to the corresponding output.

this.send([ msg1 , msg2 ]);

Multiple messages

It is possible to send multiple messages to a particular output by passing an array of messages within this array:

this.send([ [msgA1 , msgA2 , msgA3] , msg2 ]);

Closing the node

Whenever a new flow is deployed, the existing nodes are deleted. If any of them need to tidy up state when this happens, such as disconnecting from a remote system, they should register a listener on the close event.

this.on('close', function() {
    // tidy up any state

If the node needs to do any asynchronous work to complete the tidy up, the registered listener should accept an argument which is a function to be called when all the work is complete.

this.on('close', function(done) {
    doSomethingWithACallback(function() {

Since Node-RED 0.17

If the registered listener accepts two arguments, the first will be a boolean flag that indicates whether the node is being closed because it has been removed entirely, or that it is just being restarted. It will also be set to true if the node has been disabled.

this.on('close', function(removed, done) {
    if (removed) {
        // This node has been disabled/deleted
    } else {
        // This node is being restarted

Timeout behaviour

Since Node-RED 0.17

Prior to Node-RED 0.17, the runtime would wait indefinitely for the done function to be called. This would cause the runtime to hang if a node failed to call it.

In 0.17 and later, the runtime will timeout the node if it takes longer than 15 seconds. An error will be logged and the runtime will continue to operate.

Logging events

If a node needs to log something to the console, it can use one of the following functions:

this.log("Something happened");
this.warn("Something happened you should know about");
this.error("Oh no, something bad happened");

// Since Node-RED 0.17
this.trace("Log some internal detail not needed for normal operation");
this.debug("Log something more details for debugging the node's behaviour");

The warn and error messages also get sent to the flow editor debug tab.

Setting status

Whilst running, a node is able to share status information with the editor UI. This is done by calling the status function:


The details of the status api can be found here.

Custom node settings

A node may want to expose configuration options in a user’s settings.js file.

The name of any setting must follow the following requirements:

  • the name must be prefixed with the corresponding node type.
  • the setting must use camel-case - see below for more information.
  • the node must not require the user to have set it - it should have a sensible default.

For example, if the node type sample-node wanted to expose a setting called colour, the setting name should be sampleNodeColour.

Within the runtime, the node can then reference the setting as RED.settings.sampleNodeColour.

Exposing settings to the editor

Since Node-RED 0.17

In some circumstances, a node may want to expose the value of the setting to the editor. If so, the node must register the setting as part of its call to registerType:

RED.nodes.registerType("sample",SampleNode, {
    settings: {
        sampleNodeColour: {
            value: "red",
            exportable: true
  • value field specifies the default value the setting should take.
  • exportable tells the runtime to make the setting available to the editor.

As with the runtime, the node can then reference the setting as RED.settings.sampleNodeColour within the editor.

If a node attempts to register a setting that does not meet the naming requirements an error will be logged.