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Reactive Web Components (RWC)


rwc (BETA)

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RWC is a unique mix of Shadow DOM + Virtual DOM + Redux to create web-components. This approach is an attempt to find a balance between a scalable paradigm and performance.


npm install rwc --save


  • Small footprint (~100 SLOC) with no dependencies at all.
  • Reactive approach — actions trigger an update on the state which triggers view updation.
  • Can integrate with multiple virtual dom implementations like preact or snabbdom simulataneously.
  • Caches event handler between renders.
  • Passes proper JS objects to components using props.
  • Support for custom side-effects using Tasks.
  • Uses shadow dom v1 API.


Components are composed of three functions —

  • init(component) : The function takes in the current instance of the component and returns the initial state.
  • update(state, action): A reducer function like that in Redux that takes an input state and based on the action returns a new output state.
  • view(state, dispatch): The view function converts the state into a virtual DOM tree. Additionally it also gets a dispatch() function as an argument which can dispatch custom actions.

All these three functions are essentially pure functions, ie. they have no side effects and should remain referentially transparent for all practical purposes.

Create Component

// CounterComponent.js

import h from 'snabbdom/h'

// Creates an initial state
const init = () => {
  return {count: 0}

// Reducer function like that in redux
const update = (state, {type, params}) => {
  switch (type) {
    case 'INC': return {count: state.count + 1}
    case 'DEC': return {count: state.count - 1}
    default: return state

// Creates virtual DOM elements
const view = (state, dispatch) => {
  return h('div', [
    h('h1', [state.count]),
    h('button', {on: {click: dispatch('INC')}}, ['Increment']),
    h('button', {on: {click: dispatch('DEC')}}, ['Decrement'])

export default {init, view, update}

Register Web Component

import rwc from 'rwc'
import CounterComponent from './CounterComponent'

function virtualDOMPatcher (shadowRoot) {
  return (vnode) => {/* patches the shadowRoot with vNode */}

// create prototype object
const ReactiveProto = rwc.createWCProto(virtualDOMPatcher, CounterComponent)

// register as usual
document.registerElement('x-counter', {prototype: ReactiveProto})

Virtual DOM Patcher

The virtualDOMPatcher function argument gives the to ability to customize how the shadow DOM is updated.


  1. view snabbdom demo

    import snabbdom from 'snabbdom'
    import h from 'snabbdom/h'
    const patch = snabbdom.init([
    function virtualDOMPatcher (shadowRoot) {
      // create wrapper element
      let __vNode = shadowRoot.appendChild(document.createElement('div'))
      return function (vNode) {
        __vNode = patch(__vNode, h('div', [vNode]))
  2. view preact demo

    import { render } from 'preact'
    import { createElement as h } from 'preact-hyperscript'
    function virtualDOMPatcher (shadowRoot) {
      let __vNode
      return function (vNode) {
        __vNode =render(vNode, shadowRoot, __vNode)
  3. view maquette demo

    import {h, createProjector} from 'maquette'
    function virtualDOMPatcher (root) {
      let __vNode
      const projector = createProjector()
      const render = () => projector.append(root, () => __vNode)
      return function (vNode) {
        if(!__vNode) {
          __vNode = vNode
        __vNode = vNode

RWC Tasks

Web components can have custom side-effects using Tasks. A task is a simple object which has a run() function. This function gets called by RWC with the current component as the first param and the dispatch function as the second. Tasks have to be created inside the update() function and returned as a part of the tuple's second value as shown below. For instance a counter web component might want to dispatch an event whenever the counter changes. This can be done by creating an event dispatching task.

// Create a event dispatching task
export class DispatchEventTask {
  constructor (name, detail) {
    this.detail = detail
    this.name = name
  run (component) {
    const ev = new CustomEvent(this.name, {
      detail: this.detail,
      bubbles: true

export const update = (state, {type, params}) => {
  switch (type) {

    case 'INC':
      // Return a tuple of state + task
      return [
        {count: state.count + 1},
        new DispatchEventTask('count-changed', _state.count)

    case 'DEC':
      const _state = {count: state.count - 1}
      return [
        new DispatchEventTask('count-changed', _state.count)
    default: return state

@@rwc actions

RWC dispatches custom actions during the lifecycle of the web component, which can be used inside the update() function.

  • @@rwc/created: Fired when the web component is initialized. The params for this action is the instance of the web component.
  • @@rwc/attached: Dispatched when the web component is inserted into the DOM. This is a good time to call something like params.getBoundingClientRect() to get the dimensions of the web component and keep it in the state.
  • @@rwc/detached: Dispatched when the component is removed from the DOM.
  • @@rwc/attr/<attr name>: This is fired whenever a web component's attribute is changed. The param is the current value of the attribute.
  • @@rwc/prop/<prop name>: Attributes have a limitation of passing data that is of string type only. For this purpose you can predefine some props that rwc will attach hooks on and whenever they are changed, this particular action will be fired.

    // create a list of props
    const props = ['aa', 'bb']
    // pass props to the factory function
    const proto = rwc.createWCProto(patcher, {update, init, view, props})
    // an instance is automatically created by the browser
    const wc = Object.create(proto)
    // this will fire action {type: '@@rwc/prop/aa', param: wc}
    wc.aa = new Date()

Mutating DOM Events

Might get deprecated in favour of tasks

In certain scenarios one might want to call preventDefault() or stopPropagation() on the events that are emitted. This can be easily done by passing additional params to the dispatch function in the view.

const view = (state, dispatch) =>
  h('div', [
    h('a', {
      href: '/example.com',
      on: {click: dispatch('CLICK', {preventDefault: true, stopPropagation: true})}
      }, [
      'Click Me!'

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Created 5 months ago

Last updated 13 days ago

npm Dependencies

window-or-global ^1.0.1

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