Like we said, Merkur is tiny and extensible library and allows you to define custom widget plugins as well. Widget plugin has two lifecycle methods, setup and create. Both methods are called during the creation of a widget; first setup, then create. Both methods receive widget and widgetProperties and must return an extended widget object.

The setup method is for creating a new widget interface and a base widget properties in widget.$in property. The create method is for connecting more plugins or reusing some plugin methods from another plugin.

We are creating custom debug plugin.

import { assignMissingKeys, createMerkurWidget, hookMethod, isFunction } from '@merkur/core';
import { componentPlugin } from '@merkur/plugin-component';
import { eventEmitterPlugin } from '@merkur/plugin-event-emitter';
import { errorPlugin } from '@merkur/plugin-error';

export function debugPlugin() {
  return {
    async setup(widget, widgetProperties) {
      assignMissingKeys(widget, {
        debug(widget) {
'State', widget.state);
'Props', widget.props);

      return widget;
    async create(widget, widgetProperties) {
      Object.keys(widget).forEach((methodName) => {
        if (!isFunction(widget, methodName)) {

        hookMethod(widget, methodName, (widget, originalMethod, ...args) => {
 `call ${key} method with `, ...args);

           // originalMethod has auto bind widget to first parameter
           return originalMethod(...args);

        // similar without hookMethod
        // originalMethod = widget[key];
        // widget[key] = (widget, ...args) => {
        //`call ${key} method with `, ...args);

        //   return originalMethod(widget, ...args);
        // };

      return widget;

export const widgetProperties = {
  $plugins: [componentPlugin, eventEmitterPlugin, debugPlugin, errorPlugin],
  // ... other properties

const widget = createMerkurWidget(widgetProperties);

We created debugPlugin which adds debug method in the setup lifecycle method and adds tracking to all widget methods in the create lifecycle method. We used our new plugin in the $plugins property of a specific widget.

Merkur has only several predefined plugins componentPlugin, eventEmitterPlugin, httpClientPlugin, errorPlugin and graphqlClientPlugin.