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Developing FlexDash/Node-RED Widgets

Planned outline (sorry):

  • the three parts of a FlexDash widget: the widget (.vue file), the node (.js file), the config (.html file)
  • very brief primer on a .vue file: template, component, styles
  • very brief primer on a Vue component: props, data, computed, ... , links to Vue tutorials
  • FlexDash extensions to Vue components: help, tips, output, ...
  • connecting a node with its widget
  • WidgetAPI: communicating with the widget
  • working with props: maintaining JSON data structures, using arrays, ...
  • configuring the widget
  • automatically generating the node and/or the configuration

When talking about FlexDash widgets and nodes it is important to understand that there are three distinct parts to a widget and its integration into Node-RED:

  1. The widget is a piece of code found in a .vue file that implements the representation of the widget in the dashboard. This code runs in the browser within the FlexDash framework.
  2. The node is a piece of code found is a .js file that implements the machinery supporting the widget in Node-RED. It receives Node-RED messages, potentially transforms them, and forwards them to the corresponding widget in the dashboard. It also can receive messages from the widget and outputs them as Node-RED message. This code runs in Node-RED, specifically in Node.js on "the server".
  3. The node config is a piece of code and html found in a .html file that implements the editing and configuration of the node in the Node-RED flow editor (also called admin). It provides the property sheets letting the user configure widget props, etc. This code is served by Node-RED and runs in the browser when editing flows.

Note: in normal Node-RED terminology both the .js and .html are called "the node". They're distinguished here to make it clear what runs where.

Options for writing custom widgets

FlexDash and its integration into Node-RED supports the development of custom widgets at several levels of flexibility.

In the future, the simplest way to get started will be to use the not yet implemented custom widget node. This node is akin to the ui_template node in the std Node-RED dashboard. It will have an editor panel where a FlexDash widget's source file can be created and dynamically pushed to the dashboard.

The next level is to create an NPM package containing a widget and the corresponding Node-RED node. One of the benefits over the custom widget node is to be able to publish the result as NPM package allowing other Node-RED users to install it. Another benefit is that the widget can import dependencies, e.g. NPM packages that it needs to produce a visualization.

Creating the NPM package is very similar to writing a custom Node-RED node except that the node part can be auto-generated. It is recommended to start by writing the widget, a .vue file, and then running a generator that produces the corresponding node .js and node config .html files. All the nodes in node-red-fd-corewidgets have been produced this way.

Once the generator has been run everything can be tested using the "flexdash dev server" built into node-red-flexdash. The dev server automatically pushes edits of the .vue file to FlexDash as soon as the file is modified and it enables all the in-browser Vue debugging tools.

When the widget is ready for "production use" a build step generates the files necessary to load the widget into FlexDash without the dev server. This also enables the widget and its associated node to be distributed via NPM to other Node-RED users.

The auto-generated code more or less forwards incoming messages straight to the widget's props. For some complex widgets it may be necessary to write a custom node instead of using the code generator. This can be done relatively easily by starting from the generated code and modifying it. The generated code is relatively simple and primarily calls helper functions in node-red-flexdash. Depending on what is desired, the custom node code can choose to call the same helper functions or not. The dev server and build step options remain the same.

Vue primer

FlexDash uses the very popular Vue 3 web framework. Beyond its popularity, which ensures that there are many tutorials and examples out there, the benefit of Vue is that it has a relatively soft learning curve and that it provides some structure for the UI code.

Vue uses "enhanced" web components. A web component is a piece of code that gets instantiated on a web page using an HTML tag. E.g., just like <h1>...</h1> produces a heading on the page writing <my-widget>...</my-widget> can produce an instance of a Vue component.

To write a Vue component up to three parts are necessary: 1. An HTML template: this is a fragment of HTML that replaces <my-widget>...</my-widget> or put differently, this is the HTML that renders the component. 2. A piece of javascript code: this is what implements the dynamic functions of the component. 3. A CSS style sheet that provides styling for the component's HTML.

One nice aspect of Vue is that is allows these three parts to be combined into a single file (this is called Single File Components, or SFC, in Vue) and the file extension is .vue.

(Note that technically of the three parts above only the javascript code is required but in practice the vast majority of FlexDash widgets have an HTML template and many have some CSS.)

In addition to using Vue 3 FlexDash also uses the Vuetify 3 toolkit. Vuetify is essentially a web component library that provides an implementation of many typical UI components such as buttons, data tables, menus, forms, test fields, icons, etc.

A very simple widget

A FlexDash widget is a Vue component that follows some additional conventions and that is flagged to FlexDash as being a widget.

There are many excellent Vue tutorials (see ...), the intent of the example in this section is to complement these by showing a complete FlexDash widget example that uses some of the conventions used by FlexDash and that explains how all the pieces, including the Node-RED ones, hang together. The example used is a simple-button widget displays a button with a custom label and that outputs a value when clicked.

For the sake of formatting the HTML, CSS Javascript portions of the .vue file are shown below separately. They are simply concatenated one after another in the actual .vue file.

The HTML of a widget is inserted into a "card" by FlexDash, so the widget only needs to display the content of that card.

  <v-btn variant="elevated" class="ma-auto" @click="clicked()">
    <span class="label">{{ label }}</span>

The template here consists of a single HTML tag: v-btn, which is a button component provided by Vuetify ( all Vuetify components start with "v-"). The elevated variant has a drop shadow giving the button an elevated look and "ma-auto" is a Vuetify helper class that adds a margin: auto CSS tag to the component, which causes it to be vertically and horizontally centered in its HTML container, which is the above-mentioned card.

Within the button comes the text the button displays, which is the content of a (yet to be defined) variable called label. The {{..}} notation is used to insert the output of a Javascript fragment, which here is just the content of a variable.

The @click is a Vue notation that causes the code on the right of the = to be executed when the component emits an event called 'click'. In this example a (yet to be defined) method 'clicked' is called.

The Javascript for the simple-button widget is:

<script scoped>
export default {
  name: 'SimpleButton', // the name of the widget
  // help text shown in the Node-RED info pane of the node
  help: `Button to send a message
Pressing the button sends a message with a specified payload.
The button may contain a label that is shown centered in the widget.`,

  // props are the inputs to the widget, they can all be set from Node-RED
  props: {
    label: { default: "clickme", tip: "" },
    output: { default: "I was clicked" },

  output: true, // signals to FlexDash that the widget can output a message

  // simple methods within the component
  methods: {
    clicked() { // handle the clicking of the button, i.e., the handler for the '@click'
      this.$emit('send', this.output)

The CSS for the simple button widget is:

<style scoped>
  .label { color: red; }

The resulting widget looks as follows in FlexDash:

In order to work with Node-RED the node .js and .html files can be auto-generated. While these are relatively short only the fragments that tie everything together are shown here.

The .html file displays the editing panels for the widget in the Node-RED flow editor:

The .js file implements the node running in the Node-RED run-time:

Background on linking in dev mode vs. production

Developing widgets using the dev server vs. running the same widgets in "production" uses quite different linking steps and this can lead to rather confusing errors and problems. The linking steps in Javascript involve a number of tools, plugins to those tools and everything is both complicated and magic. When the magic fails one is left with a heap of complexity leading to frustration...

Linking external packages with custom widgets is fundamentally different in dev mode from production mode. In dev mode everything is served up by Vite ("dev server"). It serves up one source tree, which is the FlexDash source tree and the external packages are essentially npm installed into that source tree (using symlinks). This means that when some module in the external package has an import statement the path is resolved by Vite the same way it resolves it for all of FlexDash.

In production mode, FlexDash is built and bundled, and then at run-time loaded into the browser. External packages with custom widgets are similarly built and bundled in isolation. They cannot simply import modules that are a part of FlexDash like in dev mode. They also cannot declare Vue, Vuetify as their own dependencies because that would result in multiple copies of these libraries loaded in the browser. The solution used is for FlexDash to export selected functions of Vue, Vuetify, and uPlot in the window global variable and to use vite-plugin-externals to transparently import references to Vue, Vuetify, and uplot.

Specifics for imports in dev mode

  • import ... from "Vue", import from "Vuetify", import from "uplot": ???
  • use of <v-some-component> is handled by the Vite vue-loader plugin and the source code is transformed to an import of vuetify/lib/components/VSomeComponent/index.mjs

Specifics for imports in production mode

  • import ... from "Vue", import from "Vuetify", import from "uplot" are transformed by the vue-plugin-externals Vite plugin to reference window.Xxxx instead of performing an actual import. These globals are set in FlexDash's main.js.
  • use of <v-some-component> magically works because all Vuetify components are registered with Vue in FlexDash's main.js, so they don't need to be explicitly imported.

Note that the explicit import of all Vuetify components is not just necessary for the registration but also to ensure they all actually show up in the bundle since FlexDash itself doesn't use all. It would make sense to exclude some very large components in the future to reduce code bloat.


Common (dev&prod) requirements

Widget files and names

  • The custom widgets must be in an NPM module whose name starts with node-red-fd-.
  • Each widget must be in a .vue file located in a widgets subdirectory in the module directory.
  • The widget should have a compound name (i.e. with a -) to avoid clashes with HTML tags.
  • The widget's file name should (must?) be the widget's name (plus the .vue extension).
  • The widget must declare its name using the CamelCase version (i.e. `name: "WidgetName" field).

Node-RED nodes

  • Node-RED nodes can be generated from the widget '.vue' file using the gen-widget-nodes.js script in node-red-flexdash. The .js and .html files can also be written manually.
  • The correspondence between a node and a widget is established by the initWidget call in the node's constructor, e.g. const widget = RED.plugins.get('flexdash').initWidget(this, config, 'WidgetName').
  • Multiple nodes can instantiate widgets of the same type, however, a node cannot instantiate multiple widgets (except for arrays), because the widget uses the node's ID and multiple calls to initWidget would create an ID clash.


  • The widget code can call Vue globals using an implicit global Vue variable.
  • The widget can use any Vuetify component in its template without special declaration or import.
  • The widget can call any Vuetify global using an implicit global Vuetify variable.
  • The widget can use uPlot using a global uplot.
  • Any other npm package must be decalred as dependency in a package.json in the widgets directory and must be installed there using npm install resulting in a ./node_modules/<package>.


  • The module with the widgets must be installed using npm. This can either be an installation from the npm repository (e.g. npm install node-red-fd-my-widgets) or an installation from source (e.g. npm install /home/myself/src/node-red-fd-my-widgets).
  • The module may be installed in the Node-RED user_dir's node_modules directory (/usr/src/node-red/node_modules when using docker, ~/.node-red/node_modules otherwise) or in the data_dir's node_modules (/data/node_modules with docker, ~/.node-red/???/node_modules otherwise).
  • node-red-flasdash ends up searching for widget vue files in:
  • ${user_dir}/node_modules/node-red-fd-*/widgets/*.vue
  • ${user_dir}/node_modules/@*/node-red-fd-*/widgets/*.vue
  • ${data_dir}/node_modules/node-red-fd-*/widgets/*.vue
  • ${data_dir}/node_modules/@*/node-red-fd-*/widgets/*.vue

Requirements for development

There are no additional requirements for development other than having to run the "dev server" using the flexdash dev server node in node-red-flexdash.

In dev mode (using the dev server) all Javascript pieces are served up by the dev server (Vite) using modern ESM modules. Vite does some pre-bundling and other optimizations to be able to serve everything up relatively quickly. All the cached modules can be found in flexdash-src/node_modules/.vite, i.e. in the FlexDash source directory installed by the dev server or by yourself. There are scenarios where an rm -rf of that directory helps...

When the dev server is started it prints some info messages to the Node-RED log about custom widgets it has loaded. E.g.:

18 Aug 13:09:02 - [info] [flexdash dev server:FD] widgets: searching in /usr/src/node-red
18 Aug 13:09:02 - [info] [flexdash dev server:FD] widgets: searching in /data
18 Aug 13:09:02 - [info] [flexdash dev server:FD] widgets: found node-red-fd-testnodes
18 Aug 13:09:02 - [info] [flexdash dev server:FD] widgets: found node-red-fd-network-diagram

Requirements for production

In production FlexDash is served up using an optimized and minified bundle. External/custom widgets are imported "on the side" from their own bundle. This loading is done by the palette loader, which obtains a handle onto the laoded widget bundle and inspects it to determine what to add to FlexDash's palette. The dashboard config sent to FlexDash contains the names of widgets and these names are looked up in the palette, that's how FlexDash knows to display a custom widget.

Often widgets need to call code that exists in FlexDash, such as Vue, Vuetify, or uPlot functions. These are accessed via global variables and the build must be configured to exclude these libraries from the build of the widget module (see below).

  • The package.json in the widgets directory should have a build script which invokes vite build. This allows the build step to be invoked using npm run build.
  • The widgets directory must have a vite.config.js to configure the build step, it is best to adapt the config from the node-red-fd-testnodes repository. (Primarily, any dependencies on other npm modules must be added.)
  • The widgets directory must have an index.js file with a line that re-exports all widgets: export default import.meta.globEager("./*.vue"), see the node-red-fd-testnodes repo.
  • The build must be performed before Node-RED is started, resulting in a dist/ file below the widgets directory. Additionally, if the .vue files include <style> a dist/style.css file will be present.

When Node-RED starts and the node-red-flexdash dashboard node starts it prints some info messages to the Node-RED log about extra widget modules it has found. E.g.:

18 Aug 13:09:05 - [info] [flexdash dashboard:FD] Looking for extra widgets in /usr/src/node-red,
18 Aug 13:09:05 - [info] [flexdash dashboard:FD] Extra widget modules: @flexdash/node-red-fd-tes
tnodes/widgets/dist/, @tve/node-red-fd-network-diagram/widgets/dist/