iTwin.js Extensions

An iTwin.js Extension is a separate JavaScript module that can load on demand into an iTwin.js frontend application. The separate deliverable enables Extensions to provide extensibility of an iTwin.js application without having to re-bundle the application. Extensions have access to a limited subset of iTwin.js functionality through @itwin/core-extension to enable seamless integration with the host app.

What can extensions do?

Extensions can be used for many different purposes, such as:

  • Add a new decorator to an existing application to better support your custom workflows.
  • Write event based processing, i.e., subscribe to an iModel Event, or Unified Selection Event, and process that change.

How to get started

An iTwin.js Extension at a minimum is a single JavaScript file and a manifest (i.e., a package.json file with some additional properties). To get started, create a new directory for the Extension.

Setup the Manifest

The first step to creating an extension is to create a manifest. Create the package.json by running, in the directory created above:

npm init --yes

The following properties must be added to the package.json file:

  • Name: the name of the Extension.
  • Version: the version of the Extension in the format x.x.x.
  • Main: where to find the javascript file.
  • ActivationEvents: events that define when the iTwin.js application should execute your Extension. Currently, we only support onStartup, which will execute the Extension as soon as it is added to the application.

Here is a minimal example:

// package.json
  "name": "my-new-extension",
  "version": "0.0.1",
  "main": "./dist/index.js",
  "type": "module",
  "activationEvents": [

Next, you'll want to add TypeScript, and the required dependencies for developing with the iTwin.js shared libraries:

  // package.json
  "dependencies": {
    "@itwin/core-extension": "^3.2.0"
  "devDependencies": {
    "typescript": "~5.0.2",
    "@itwin/build-tools": "^3.2.0",

A basic tsconfig.json file needs to be setup for development. Create a new tsconfig.json file next to the package.json with the following contents:

// tsconfig.json
  "extends": "./node_modules/@itwin/build-tools/tsconfig-base.json",
  "include": ["./*.ts", "./*.tsx"]

Next, add your favorite JavaScript tool to bundle your code together. Bundling is the process of combining multiple small source files into a single file. Bundling is necessary because when Extensions are loaded by the host iTwin.js application, they can only load and execute one file at a time. It is also a good idea to make the file as small as possible, a process known as minification. For JavaScript, popular bundlers are rollup.js, esbuild, and webpack. Here is an example using esbuild:

npm i --save-dev esbuild @esbuild-plugins/node-modules-polyfill @esbuild-plugins/node-globals-polyfill

Add the following entry into the scripts section in package.json to build the final bundle:

// package.json
  "scripts": {
    "build": "node esbuild.js"

And finally, an esbuild configuration file (esbuild.js) should be placed next to the package.json. The configuration tells esbuild to bundle and minify the files, as well as adds some necessary polyfills:

// esbuild.js
import { NodeModulesPolyfillPlugin } from "@esbuild-plugins/node-modules-polyfill";
import { NodeGlobalsPolyfillPlugin } from "@esbuild-plugins/node-globals-polyfill";
import path from "path";
import esbuild from "esbuild";
import { fileURLToPath } from "url";
import { argv } from "process";

const dir = path.dirname(fileURLToPath(import.meta.url)).replace(/\\/g, "/");
const arg = argv.length > 2 ? argv[2] : undefined;

    entryPoints: ["src/index.ts"],
    bundle: true,
    minify: true,
    define: { global: "window", __dirname: `"${dir}"` },
    outfile: "dist/index.js",
    plugins: [new NodeGlobalsPolyfillPlugin(), new NodeModulesPolyfillPlugin()],
    format: "esm",
    loader: {
      ".svg": "dataurl",
      ".woff": "dataurl",
      ".eot": "dataurl",
      ".ttf": "dataurl",
      ".woff2": "dataurl",
      ".cur": "dataurl",
      ".png": "dataurl",
  .catch(() => process.exit(1));

Creating the Extension

The only requirement when creating an Extension is to define a default function. The default function will execute on the Extension's activation event. The function would run immediately if onStartup was specified as the Activation Event. For example:

// src/index.ts
export default function main() {
  console.log("Hello from Extension!");

The above would print "Hello from Extension!".

We can extend functionality by adding, for example, adding some decorators representing IoT devices, and when clicked will navigate to a specific view. First, we will query for all ceilings, walls, windows, etc, and hide them from the view. Then, we will query for all IoT devices, and add a decorator for each one.


import { ExtensionHost, QueryRowFormat, ScreenViewport } from "@itwin/core-extension";
import { SmartDeviceDecorator } from "./SmartDeviceDecorator";

export class IotMarkerExtension {
  public static start = () => {
    ExtensionHost.viewManager.onViewOpen.addOnce(async (vp: ScreenViewport) => {
        viewflags: {
          visEdges: false,
          shadows: false,

      const categoriesToHide = [
        "'Wall 2nd'",
        "'Wall 1st'",
        "'Dry Wall 2nd'",
        "'Dry Wall 1st'",
        "'Brick Exterior'",
        "'WINDOWS 1ST'",
        "'WINDOWS 2ND'",
        "'Ceiling 1st'",
        "'Ceiling 2nd'",
        "'light fixture'",

      const query = `SELECT ECInstanceId FROM Bis.Category WHERE CodeValue IN (${categoriesToHide.toString()})`;

      const result = vp.iModel.createQueryReader(query, undefined, {
        rowFormat: QueryRowFormat.UseJsPropertyNames,

      const categoryIds = [];
      for await (const row of result)

      vp.changeCategoryDisplay(categoryIds, false);

      ExtensionHost.viewManager.addDecorator(new SmartDeviceDecorator(vp));


import {
} from "@itwin/core-extension";
import { SmartDeviceMarker } from "./SmartDeviceMarker";

export class SmartDeviceDecorator implements Decorator {
  private iModel: IModelConnection;
  private markerSet: Marker[];

  constructor(vp: ScreenViewport) {
    this.iModel = vp.iModel;
    this.markerSet = [];

  private async getSmartDeviceData() {
    const query = `
      SELECT  SmartDeviceId,
              FROM DgnCustomItemTypes_HouseSchema.SmartDevice
              WHERE Origin IS NOT NULL

    const results = this.iModel.createQueryReader(query, undefined, {
      rowFormat: QueryRowFormat.UseJsPropertyNames,

    const values = [];
    for await (const row of results) {
    return values;

  private async addMarkers() {
    const values = await this.getSmartDeviceData();

    values.forEach((value) => {
      const smartDeviceMarker = new SmartDeviceMarker(
        { x: value.origin.x, y: value.origin.y, z: value.origin.z },
        { x: 40, y: 40 },


  public decorate(context: DecorateContext): void {
    this.markerSet.forEach((marker) => {


import {
} from "@itwin/core-extension";
import { XAndY, XYAndZ } from "@itwin/core-geometry";

import Bed from "../assets/Bed.png";
import DishWasher from "../assets/DishWasher.png";
import Garage from "../assets/Garage.png";
import Jacuzzi from "../assets/Jacuzzi.png";
import Light from "../assets/Light.png";
import Lock from "../assets/Lock.png";
import Oven from "../assets/Oven.png";
import Speaker from "../assets/Speaker.png";
import Thermostat from "../assets/Thermostat.png";
import TV from "../assets/TV.png";
import Washer from "../assets/Washer.png";

const getIcon = (iotType: string) => {
  switch (iotType) {
    case "Bed":
      return Bed;
    case "DishWasher":
      return DishWasher;
    case "Garage":
      return Garage;
    case "Jacuzzi":
      return Jacuzzi;
    case "Light":
      return Light;
    case "Lock":
      return Lock;
    case "Oven":
      return Oven;
    case "Speaker":
      return Speaker;
    case "Thermostat":
      return Thermostat;
    case "TV":
      return TV;
    case "Washer":
      return Washer;
      return "";

export class SmartDeviceMarker extends Marker {
  private elementId: string;

    location: XYAndZ,
    size: XAndY,
    _smartDeviceId: string,
    smartDeviceType: string,
    elementId: string,
  ) {
    super(location, size);
    this.elementId = elementId;

    const image = new Image();
    image.src = getIcon(smartDeviceType);

  public override onMouseButton(ev: BeButtonEvent): boolean {
    if (!ev.isDown)
      return true;
    const vp = ExtensionHost.viewManager.selectedView;
    if (!vp)
      return true;
    vp.zoomToElements(this.elementId, {
      animateFrustumChange: true,
      standardViewId: StandardViewId.RightIso,
    return true;


import { IotMarkerExtension } from "./IotMarkerExtension";

export default function main() {
  console.log("Hello from Extension!");
  console.log("Custom Markers Registered!");

The final file structure should look something like this:

│   package.json
│   esbuild.js
│   tsconfig.json
│   │   index.ts
│   │   IotMarkerExtension.ts
│   │   SmartDeviceDecorator.ts
│   │   SmartDeviceMarker.ts
    │   index.js

Loading an Extension into an iTwin.js Application

Extensions need to be served somewhere so that the iTwin.js application can load the Extension at runtime.

A useful way to serve JavaScript locally is to add serve as a dev dependency npm i --save-dev serve, then adding a script to your package.json: "serve": "serve . -p 3001 --cors".

By default, every IModelApp has an instance of the ExtensionAdmin. The ExtensionAdmin controls the loading and execution of Extensions. An Extension must be added to ExtensionAdmin through an ExtensionProvider before it can be executed.

In the following example we add an Extension served at localhost:3001 through a RemoteExtensionProvider. You can also load Extensions locally as if they were npm packages through the LocalExtensionProvider.

const extensionProvider = new RemoteExtensionProvider({
  jsUrl: "http://localhost:3001/dist/index.js",
  manifestUrl: "http://localhost:3001/package.json",

The next step is to register your host with the ExtensionAdmin. The ExtensionAdmin will only load Extensions from registered hosts.


The last step is to add the Extension to the ExtensionAdmin. Once the Extension has been added, its default function will immediately execute if the onStartup Activation Event was defined in the manifest.

  .catch((e) => console.log(e));

Last Updated: 18 August, 2023