A Vainilla Library for Web Components

Open in Visual Studio Code npm license npm npm npm CodeQL Tests

Why "MichiJS?"

MichiJS React StencilJS SvelteJS VanillaJS
Prefer real DOM over virtual DOM
Prefer Javascript templates over compiled plain text
Templates with JSX
Element internals support
Does not require extensions to be identified by the IDE
Differentiation between attributes and properties in jsx / templates
Standard Web Components
Observables / stores support
Esbuild as default bundler
TypeScript support
Styling / Constructable Stylesheets support
Automatic component type generation
Without polyfills
Attributes / Native events support
Supports Shadow DOM
Supports Custom Built-in elements
Can be used with different frameworks right out of the box
✅ = implemented
⭕ = partially implemented
❌ = not implemented

Getting Started

You can use this template or you can see on Code Sandbox.

Creating components

MichiJS custom elements are plain objects.

New components can be created using the jsx/tsx extension, such as MyCounter.tsx.

import { createCustomElement, EventDispatcher } from "@michijs/michijs";
import { counterStyle } from "./counterStyle";

export const MyCounter = createCustomElement('my-counter', {
  reflectedAttributes: {
    count: 0
  methods: {
    decrementCount() { this.count-- },
    incrementCount() { this.count++ },
  events: {
    countChanged: new EventDispatcher<number>()
  adoptedStyleSheets: [counterStyle],
  observe: {
    count() {
  render() {
    return (
        <button onpointerup={this.decrementCount}>-</button>
        <button onpointerup={this.incrementCount}>+</button>

Note: the .tsx extension is required, as this is the standard for TypeScript classes that use JSX.

To use this component, just use it like any other HTML element:

import '../Counter';

<my-counter oncountchanged={(ev) => console.log(`New count value: ${ev.detail}`)} />

Or if you are using jsx

import Counter from '../Counter';

<Counter oncountchanged={(ev) => console.log(`New count value: ${ev.detail}`)} />

Component structure

A component consists of the following properties:

Property Description
attributes Allows to define attributes.
nonObservedAttributes Allows to define non observed attributes. This is useful for complex objects that cannot be observed.
reflectedAttributes Allows to define reflected attributes and follows the Kebab case. A reflected attribute cannot be initialized with a true value
transactions Transactions are functions that notify changes at the end of the transaction.
methods Methods are functions that notify changes at the time of making the change.
adoptedStyleSheets Allows to use Constructable Stylesheets. Remember that you need to use Shadow DOM to be able to use Constructable Stylesheets. In case your component doesn't support this feature, it will return a style tag.
cssVariables Allows to define CSS variables. CSS variables changes does not trigger a rerender.
reflectedCssVariables Allows to define reflected CSS variables and follows the Kebab case. CSS variables changes does not trigger a rerender. A reflected CSS variable cannot be initialized with a true value
computedStyleSheet Allows you to define a Constructable Stylesheet that depend on the state of the component. When there is no shadow root the style will be reflected in the style attribute.
render Function that renders the component.
observe Contains methods with a name of an attribute / reflected attribute / css variables / observable like. Those methods are executed when a change has been made to their corresponding property.
Custom Element related
willConstruct This method is called at the start of constructor.
didConstruct This method is called at the end of constructor.
connected This method is called when a component is connected to the DOM.
willMount This method is called right before a component mounts.
didMount This method is called after the component has mounted.
willUpdate This method is called before re-rendering occurs.
didUpdate This method is called after re-rendering occurs.
willReceiveAttribute This method is called before a component does anything with an attribute.
didUnmount This method is called after a component is removed from the DOM.
Form-associated Custom Element related
formAssociatedCallback Called when the browser associates the element with a form element, or disassociates the element from a form element.
formDisabledCallback Called after the disabled state of the element changes, either because the disabled attribute of this element was added or removed; or because the disabled state changed on a fieldset that's an ancestor of this element. The disabled parameter represents the new disabled state of the element. The element may, for example, disable elements in its shadow DOM when it is disabled.
formResetCallback Called after the form is reset. The element should reset itself to some kind of default state. For input elements, this usually involves setting the value property to match the value attribute set in markup (or in the case of a checkbox, setting the checked property to match the checked attribute.
formStateRestoreCallback Called in one of two circumstances:
  • When the browser restores the state of the element (for example,after a navigation, or when the browser restarts). The mode argument is "restore" in this case.
  • When the browser's input-assist features such as form autofilling sets a value. The mode argument is "autocomplete" in this case.
The type of the first argument depends on how the setFormValue() method was called.
events Allows you to define an event to his parent and triggering it easily. It will be defined using Lower case. For example countChanged will be registered as countchanged.
subscribeTo Allows you to subscribe to an observable like (like a store). When the store emit an event, the custom element will be re-rendered.
shadow Allows you to add a Shadow DOM. By default, it uses open mode on Autonomous Custom elements and does not use Shadow DOM on Customized built-in elements. Only this elements are allowed to use Shadow DOM.
formAssociated This tells the browser to treat the element like a form control.
fakeRoot Allows to create a fake root on the element. This is especially useful if you don't have shadow root. Since it allows you to add children from a parent node.
extends Allows to create a Customized built-in element
tag The tag to extend
class The class you want to extend

If the extends field is not provided an Autonomous custom element will be created.

store structure

A store consists of the following properties:

Property Description
state Allows to define the store state.
transactions Transactions are functions that notify changes at the end of the transaction.

stores use proxies to listen for changes in their state, in addition, they are observable. Each component has an store to listen for changes in its state.


To use css we provide functions to create Constructable Stylesheets.


Allows to create a Constructable Stylesheet with a CSSObject

export const counterStyle = createStyleSheet({
  ':host': {
    display: 'flex',
    flexDirection: 'row'
  span: {
    minWidth: '60px',
    textAlign: 'center'


Allows to create a Constructable Stylesheet with a Template String. Recomended extension for VSCode.

export const counterStyle = css`
  :host {
      display: flex;
      flex-direction: row;

  span {
      min-width: 60px;
      text-align: center;

CSS module scripts

We do not provide support for this functionality yet as ESBuild does not support it yet. You can read how it works here



Allows to set attributes and event listeners to the host element itself.


Creates a container component without styles with the tag "michi-list"


Creates a container component without styles with the tag "michi-fragment"


(Only available if formAssociated is true)

It allows to:


Create a component whose content will load after the promise ends. In the meantime you can choose to show a load component or not show anything.

Provides the ability to move around the web page without reloading the page. It uses the same attributes as an anchor tag but also allows the use of URL objects. Uses the goTo method.

Custom element methods


Allows to get a child element from the host with the selector


Forces the element to re-render


Create unique IDs with a discernible key

Attributes vs Properties in jsx

Usually, if you want to get an html like this:

<div class='test'></div>

In React / Stencil / etc you should write a jsx like this:

() => <div className='test'></div>

And eventually code like this would be executed:

const el = document.createElement('div');
el.className = 'test';

In MichiJS you have the freedom to use both attributes and properties and the result will be the same:

// Using properties
() => <div _=></div>
// Using attributes
() => <div class='test'></div>

And eventually code like this would be executed:

const el = document.createElement('div');
// Using properties
el.className = 'test';
// Using attributes
el.setAttribute('class', 'test')

In this way the jsx syntax of MichiJS is more similar to html.

Special attributes


Indicates that their children are created but not updated


Indicates that their Children are not created or updated. Element creation/update is delegated


Callback that is called when the element is created


Callback that is called when the element is updated


There are 3 ways to create a list

Using map

It's the usual way to create lists in jsx.

const arrayTest = [0, 1, 2];

arrayTest.map(item => <div key={item}>{item}</div>)

This will generate an element like:


Why create the michi-list element? This is the way to avoid using Virtual DOM. Because the algorithm is dumb, it needs a way to remember that element is a list.

Using List component

It's similar to using maps. But it allows to use different container than michi-list.

const arrayTest = [0, 1, 2];

  renderItem={item => <div key={item}>{item}</div>}

This will generate an element like:


Using ElementList

Is a proxy that allows you to avoid using dom diff algorithms to render lists. This allows it to have a performance close to vanilla js. An operation on the data implies an operation on the associated elements.

const arrayTest = new ElementList(0, 1, 2);

  renderItem={item => <div>{item}</div>}

This will generate an element like:



Map List component ElementList
Performance Diff algorithm order Diff algorithm order Close to vanilla
Container michi-list michi-list or any other element michi-list or any other element
Keys Required Required Not required
Index Yes Yes No
Transactions allowed Yes Yes No
Updates The entire component The entire component Just the list itself


The intention of using a custom routing tool is to avoid the use of strings to represent the urls and to use modern apis that allow the use of the URL object itself. It also allows to separate the components of the routes which allows a cleaner code.

const Redirect = () => {
  // Will generate and go to this url: /sync-route
  return <></>

//Parent routes
export const { urls, Router, pages } = registerRoutes({
  syncRoute: {
    /**The component to display */
    component: <div>Hello World</div>,
    title: 'Sync title'
  //Redirect route
  '/': {
    component: <Redirect />

//Child routes
export const { urls: urlsChild, Router: RouterChild, pages: pagesChild } = registerRoutes({
  // Async route
  asyncChildRoute: {
    searchParams: {
      searchParam1: String, 
      searchParam2: Number
    hash: ['#hash1', '#hash2']
    /** The promise to wait */
    promise: async () => (await import('./AsyncChildExample')).AsyncChildExample,
    /**The title of the page */
    title: 'Async Page title'
    /**The component to display while the promise is loading */
    loadingComponent: <span>Loading...</span>
  //The parent route
}, urls.syncRoute);

urlsChild.asyncChildRoute({ searchParams: { searchParam1: 'param 1', searchParam2: 2}, hash: '#hash1' })
// Will generate this url: /sync-route/async-child-route?searchParam1=param+1&searchParam2=2#hash1

Router and RouterChild are components that represent the mount points of each registered route.

The "pages" function is a utility to create asynchronous components that includes the search params and component hashes with the types that were defined when the route was registered

export const AsyncChildExample = pagesChild.asyncChildRoute(({ searchParams, hash }) => (
      {/* Will show the value of searchParam1 */}
      {/* Will show true if the hash is #hash1 */}


It is supported by using a custom store

const translator = new I18n<'es' | 'en'>(localStorage.getItem('lang'));

const store = translator.createTranslation({
  es: () => import('./translations/es.json'),
const t = store.state.t;

export const MyComponent = createCustomElement('my-component', {
  subscribeTo: {
  render() {
    return (


Observable objects

Because some objects are not proxy compatible we limit the observable objects to:


If you REALLY need polyfills i recommend you to read this topics:

Browser Support

Customized built-in elements

Autonomous custom elements

Compatibility with frameworks

Element internals

Supporting MichiJS


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