Lego is a fast web-components library

LEGO: Modern Web-Components

LEGO (Lightweight Embedded Gluten-free Objects) is a NodeJS tool to build ๐Ÿš€ fast, โ™ป๏ธ reactive, ๐Ÿก native web-components that are easy to digest ๐ŸŒฑ for your browser.

Lego is:

Lego is not (and will never be):

View the demo and their source ๐Ÿงช.

Lego is inspired from the native Web-Component spec and Riot.

Itโ€™s just much lighter with simplicity, source that are easy to read, to hack and to maintain. The core lib is only 61 LOC! Lego is as light as 3Kb for the full bundle!

Demo: view in action


Lego is based on npm and the latest node.

You need to install the compiler with npm i @polight/lego

Quick start

Hello World

Create a file called bricks/hello-world.html:

  <p>Hello ${}</p>

  init() { this.state = { name: "World!" } }

Compile with npx lego bricks

And use it in your index.html

<script src="./dist/index.js" type="module"></script>
<hello-world />

Run a local web server (ie: python3 -m http.server) and display your index.html (ie: http://localhost:8000).

More Advanced Web-Component Example


  <div :if="state.registered">
    <h1>${state.firstName} ${state.lastName}'s profile</h1>
    <p>Welcome ${state.firstName}!</p>
    <section :if="state.fruits.length">
      <h3>You favorite fruits:</h3>
        <li :for="fruit in state.fruits">${} ${fruit.icon}</li>
    <button :if="!state.registered" @click="register">Register now</button>

  init() {
    this.state = {
      registered: false,
      firstName: 'John',
      lastName: 'Doe',
      fruits: [{ name: 'Apple', icon: '๐ŸŽ' }, { name: 'Pineapple', icon: '๐Ÿ' }]

  register() {
    this.render({ registered: confirm('Do you want to register?') })

Compile this component: npx lego bricks

Then include it in your page:


<script src="./dist/index.js" type="module"></script>

Run your webserver and see your little app!

When developing you may want to automatically watch files changes. In that case pass the -w flag: npx lego -w bricks

Tip: you probably want to store this task with a shortcut like npm run watch. To do so just add "watch": "lego -w bricks" in you package.json scripts.

Understanding Webcomponents

A component can optionally have 3 parts: some HTML in a <template> tag, some JavaScript in a <script> tag and some CSS in a <style> tag.

Template tag

An HTML template is written within a <template> tag.

These superpowers can be recognized with their : or @ prefixes.

The possible directives are:

Note that :if and :for attributes, when used in the same tag should be used with an order in mind: <a :if="user" :for="user in state.users"> wonโ€™t work, but <a :if="state.users.length" :for="user in state.users"> will first evaluate if the users array has items, or <a :for="user in users" :if="user"> will check that each individual user has a value.

<a :if="state.isAdmin" :for="user in state.users"> wonโ€™t loop at all if isAdmin is false.

:if Directive

Conditionally display a tag and its descendants.

Example: <p :if="state.count < 5">Less than 5</p> will be displayed if the condition is met.

:for Directive

Repeat a tag based on a property.

The syntax is as follow: :for="item in state.items". The item value will be available trough ${item} within the loop.

If you need an incremental index i, use :for="item, i in state.items".

Example: <li :for="attendee in state.attendees">${attendee}</li> with a state as this.state = { attendees: ['John', 'Mary'] } will display <li>John</li><li>Mary</li>

: Custom Directive

A custom directive will interpret in JS whatever you pass as value.

  <a :href="this.getUrl('144')">Visit Profile</a>
  getUrl(id) { return `/user/${id}` }


<a href="/user/144">Visit Profile</a>

Boolean attributes

Example: <input type=checkbox :checked="state.agreed" :required="state.mustAgree">. With the following state: this.state = { agreed: false, mustAgree: true } would render <input type=checkbox required="required">.

@ Directive for binding Events

  <button @click="sayHi" name="the button">click</button>

  sayHi(event) {
    alert(`${'name')} says hi! ๐Ÿ‘‹๐Ÿผ`)

Reactive Properties

The state is where the reactiveness takes place.

declare a state object in the init() function with default values:

init() {
  this.state = {
    user: { firstname: 'John', lastname: 'Doe' },
    status: "Happy ๐Ÿ˜„"

Displaying a state value is as simple as writing ${state.theValue} in your HTML.

When you need your component to react, call the this.render() method with your updated state:

itemSelected(event) {
  this.render({ selected: "apple", isAdmin: true })

This will refresh your component where needed.

When state is just mutated, the changed(changedProps) is called. This changed() method is called before (re-)rendering.

Component Attributes

Attributes declared on the components will be all be accessible through the state. If the property is initialized in the this.state, the attribute will be reactive:

<x-user status="thinking ๐Ÿค”"><x-user>

status will therefore be reactive and the thinking ๐Ÿค” attribute value will overwrite the Happy ๐Ÿ˜„ default status.

โš ๏ธ A property that is not declared in the state wonโ€™t be reactive.

These properties can be accessed through this.getAttribute() from within the component. After all, these components are just native! ๐Ÿก


Slots are part of the native web-component. Because Lego builds native web-components, you can use the standard slots as documented.



  <span>This user is in Paris</span>


  <h1>User profile</h1>
  <p>important information: <slot></slot></p>

Will write โ€ฆ<p>important information: <span>This user is in Paris</span></p>

See more advanced examples.

Reactive CSS Style

CSS is much more fun when itโ€™s scoped. Here it come with the web-components.

Here again, no trick, just the full power of web-components and scoping styles. Well, you should know that the css is reactive too! ๐Ÿ˜ฒ

Writing CSS is as easy as


  init() {
    this.state = { fontScale: 1 }

  :host {
    font-size: ${state.fontScale}rem;
  h1 {
    padding: 1rem;
    text-align: center;


:host is a native selector for web-components. It allows to select the current component itself.


You can use variables in your CSS just like in your templates.


  init() {
    this.state = { color: '#357' }
  h1 {
    color: ${ state.color };

will apply the #357 color onto h1.


LEGO_URL=</url/to/lego.min.js> npx lego <source_path> <target_file_path>

Would compile the source_path file or folder (recursively) into target_file_path js file using lego.min.js from the declared url.

As mentioned before, when developing you probably want to watch for changes with the -w option: npx lego -w <source_path> <target_file_path>

source_path: either a file or a directory (relative or absolute). If itโ€™s a directory, it will recursively read all the .html files and compile them into the target_file.

target_file_path: (default: components.js) the path (relative or absolute) to a .js file. That file will be created and contain all the components.

Naming a component

The name of the file will be the name of the component.

Example: components/x-button.html will create <x-button> component.

However in some cases you may want to give your component a different name than the file. To do so, you may give your template a name with the name attribute.



<template name="my-super-button"></template>

Will create a <my-super-button> component.

Note that because it builds native web-components, the naming convention must respect the ones from the standards (lowercase, with a dash in the name, starting with a letter, โ€ฆ)


Running tests CircleCI

Just install node dev dependencies (npm install) and run the tests (npm test).

Under the hood

Native web-components

Because Lego is actual native web-components, all its native possibilities (like slots), :host and whatever exists or will exist are on board.

Browser compatibility

Lego is based on native customElements. Support for customElement is spreading and shall increase in time.

When building a web-app you may have control of the browsers. If youโ€™re building a more general website you may need to increase the overall browser compatibility and install the custom-element polyfill.


It is still fully compatible with native custom elements. No magic behind the scene, no complexity, just a couple of useful methods to write native web-components easier.

Using a compiled component has no dependency, nothing extra injected in the browser. Compiling depends on jsdom.

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