Register Merkur widget as custom element

Merkur widget can be registered as custom element. It is helpful for use case where SSR is not important for Merkur widget. We predict that you serve your widget as single JavaScript file with defined assets.


For easy registration Merkur widget as custom element we created the @merkur/integration-custom-element module. The module is designed for only client-side.

npm i @merkur/integration-custom-element --save

How to change default Merkur template

The default Merkur template is prepared for SSR so we will remove in below sections useless parts and files to reconfigure default template to only client template. At first create new Merkur widget.

Server part

After created new Merkur widget you change your playground template for creating /server/playground/templates/body.ejs file to:

<{}></{package-name}> // something like <merkur-widget></merkur-widget>

We changed logic for reviveling widget in playground and added only custom element with name from package.json to the body part of html. The custom element auto revive Merkur widget. Now you can remove other files in /server/* folder.

CLI config

You can change merkur.config.mjs file to add @merkur/integration-custom-element/cli to extends field.

 * @type import('@merkur/cli').defineConfig
export default function () {
  return {
    extends: ['@merkur/preact/cli' ,'@merkur/integration-custom-element/cli'],

The @merkur/integration-custom-element/cli modify default @merkur/cli configuration (change playground widgetHandler to skip /widget request, turn off widget server because custom element works only in browser, turn off HMR and use hot reload instead, filter node platform tasks, force generated files to be saved to filesystem as writeToDisk = true, register css bundle plugin for including bundled css file to js).

Widget part

The default Merkur template use config npm module for resolving application environment. But config module doesn’t work in browser so we must add support for application environment to our client solution with custom element.

Create new config folder in /src/ and then there create new file /src/config/index.js where copy paste code below which add support for two environments development and production. The development environment extends production environment. So you don’t need copy all options. The webpack tree shaking logic helps removed development environment in production build.

import { deepMerge } from '@merkur/integration-custom-element';

import production from './production';
import development from './development';

let environment = null;
if (process.env.NODE_ENV === 'production') {
  environment = production;
} else {
  environment = deepMerge(production, development);

export { environment };

Now you can create your own production and development environments in /src/config/production.js and /src/config/development.js files. For example /src/config/production.js file:

export default {
  environment: 'production',
  cdn: 'http://localhost:4444',
  widget: {
    apiUrl: '',

We add our resolved environment to widget props.environment property in /src/widget.js. Same as it works in default Merkur template. The custom element don’t support Merkur slots. So we set slotFactories to empty array. Then you can remove src/components/slots folder. If you want to inline css bundle to resulted JS file then add import cssBundle from '@merkur/integration-custom-element/cssBundle' and define inlineStyle asset with cssBundle as source. At the end register your widget as custom element with registerCustomElement method which alive widget and connect widget with custom element.

/* eslint-disable no-unused-vars */
import { defineWidget } from '@merkur/core';
import {
} from '@merkur/plugin-component';
import { errorPlugin } from '@merkur/plugin-error';
import { eventEmitterPlugin } from '@merkur/plugin-event-emitter';
import { registerCustomElement } from '@merkur/integration-custom-element';

import { environment } from './config';
import View from './views/View';

import { name, version } from '../package.json';

import './style.css';

import cssBundle from '@merkur/integration-custom-element/cssBundle';

const widgetDefinition = defineWidget({
  viewFactory: createViewFactory((widget) => ({
    slotFactories: [],
  props: {
  $plugins: [componentPlugin, eventEmitterPlugin, errorPlugin],
  assets: [
      name: 'widget.css',
      type: 'inlineStyle',
      source: cssBundle,
  onClick(widget) {
    widget.setState({ counter: widget.state.counter + 1 });
  onReset(widget) {
    widget.setState({ counter: 0 });
  load(widget) {
    // We don't want to set environment into app state
    const { environment, ...restProps } = widget.props;

    return {
      counter: 0,

export default widgetDefinition;

registerCustomElement({ widgetDefinition });