Web Components

Build or maintain a web component application using $ARGUMENTS. Apply all rules below strictly.

Guiding Principles

• web standards and web platform first
• minimal external dependencies — every dependency must justify its existence
• no frameworks — use the platform: custom elements, ES modules, import maps
• no build step required for development — bundling is only for packaging third-party dependencies
• progressive enhancement over JavaScript-first design

Reference Implementation

This skill is based on the bce.design quickstarter.

Allowed Dependencies

Only these three runtime dependencies are permitted. Do not add others without explicit approval:

1. lit-html — templating and efficient DOM rendering
2. Redux Toolkit — predictable state management with unidirectional data flow
3. Vaadin Router — client-side routing via History API

Development tooling: Vite (dev server), Rollup (dependency bundling), Playwright (E2E tests).

BCE Architecture (Boundary Control Entity)

Structure code using the Boundary Control Entity pattern. Organize by feature module, not by technical layer:

app/src/
├── BElement.js              # base class for all custom elements
├── store.js                 # Redux store configuration
├── app.js                   # entry point, routing, persistence
├── app.config.js            # application metadata (name, version)
├── index.html               # single HTML entry point
├── style.css                # application styles
├── libs/                    # bundled third-party ESM modules
├── [feature-name]/          # one folder per feature module
│   ├── boundary/            # UI web components
│   ├── control/             # actions and dispatchers
│   └── entity/              # reducers and state shape
└── localstorage/
    └── control/
        └── StorageControl.js  # persistence layer

Boundary (UI Components)

• all components extend BElement — never extend HTMLElement directly
• components are the only layer that imports html from lit-html
• components dispatch actions through control functions — never dispatch directly to the store
• override view() to return a lit-html template — this is the only required method
• override extractState(reduxState) to select the relevant state slice
• access extracted state via this.state inside view()
• register every component with customElements.define('prefix-name', ClassName)
• use b- prefix for component tag names
• composite components import and compose child components

import BElement from "../../BElement.js";
import { html } from "lit-html";
import { deleteItem } from "../control/CRUDControl.js";

class ItemList extends BElement {
    extractState({ items: { list } }) {
        return list;
    }
    view() {
        return html`
        <ol>
            ${this.state.map(item => html`
                <li>${item.label} <button @click="${_ => deleteItem(item.id)}">delete</button></li>
            `)}
        </ol>
        `;
    }
}
customElements.define('b-item-list', ItemList);

Control (Actions & Dispatchers)

• define Redux actions with createAction from Redux Toolkit
• export both the action creator and a dispatcher function for each action
• dispatcher functions encapsulate store.dispatch() — boundary components call dispatchers, never dispatch directly
• use Date.now() for generating entity IDs on the client

import { createAction } from "@reduxjs/toolkit";
import store from "../../store.js";

export const itemUpdatedAction = createAction("itemUpdatedAction");
export const itemUpdated = (name, value) => {
    store.dispatch(itemUpdatedAction({ name, value }));
}

export const newItemAction = createAction("newItemAction");
export const newItem = _ => {
    store.dispatch(newItemAction(Date.now()));
}

export const deleteItemAction = createAction("deleteItemAction");
export const deleteItem = (id) => {
    store.dispatch(deleteItemAction(id));
}

Entity (Reducers & State)

• use createReducer from Redux Toolkit with builder callback
• define initialState as a plain object with sensible defaults
• use a temporal cache object pattern for form input before committing to a list
• keep state shape flat — avoid deep nesting

import { createReducer } from "@reduxjs/toolkit";
import { itemUpdatedAction, deleteItemAction, newItemAction } from "../control/CRUDControl.js";

const initialState = {
    list: [],
    item: {}
}

export const items = createReducer(initialState, (builder) => {
    builder.addCase(itemUpdatedAction, (state, { payload: { name, value } }) => {
        state.item[name] = value;
    }).addCase(newItemAction, (state, { payload }) => {
        state.item["id"] = payload;
        state.list = state.list.concat(state.item);
    }).addCase(deleteItemAction, (state, { payload }) => {
        state.list = state.list.filter(item => item.id !== payload);
    });
});

BElement Base Class

The base class provides automatic Redux integration for all components:

• connectedCallback() — subscribes to Redux store, triggers initial render
• disconnectedCallback() — unsubscribes to prevent memory leaks
• triggerViewUpdate() — extracts state, generates template, renders via lit-html
• view() — abstract, must be overridden to return a lit-html template
• extractState(reduxState) — override to select a state slice (default: entire state)
• getRenderTarget() — override to change render target (default: this)

Do not modify the BElement base class unless adding cross-cutting concerns that affect all components.

State Management

Store Configuration

import { configureStore } from "@reduxjs/toolkit";
import { load } from "./localstorage/control/StorageControl.js";
import { items } from "./items/entity/ItemsReducer.js";

const reducer = { items }
const preloadedState = load();
const config = preloadedState ? { reducer, preloadedState } : { reducer };
const store = configureStore(config);
export default store;

localStorage Persistence

• persist entire Redux state to localStorage on every store update via store.subscribe()
• use JSON.stringify / JSON.parse for serialization
• derive storage key from appName in app.config.js
• wrap save/load in try-catch — never let persistence errors crash the application
• preload persisted state on store initialization

Data Flow

User Event → Boundary Component → Control (dispatcher) → Redux Action
→ Entity (reducer) → Store → BElement.triggerViewUpdate()
→ extractState() → view() → lit-html render() → DOM

Routing

• use Vaadin Router with an HTML element as outlet
• define routes mapping paths to custom element tag names
• import all routed components before initializing the router
• use standard <a href="..."> for navigation — no programmatic routing unless necessary

import { Router } from "@vaadin/router";
const outlet = document.querySelector('.view');
const router = new Router(outlet, {});
router.setRoutes([
  { path: '/',     component: 'b-item-list' },
  { path: '/add',  component: 'b-items' }
]);

HTML Structure

• single index.html entry point with semantic HTML (<main>, <header>, <nav>, <section>, <footer>)
• declare import maps in <head> to map bare module specifiers to local bundled files
• load init.js (non-module) before app.js (module) to set up globals required by dependencies
• the router outlet is a <section> element — router replaces its content with route components

<script type="importmap">
{
  "imports": {
    "lit-html": "./libs/lit-html.js",
    "@reduxjs/toolkit": "./libs/redux-toolkit.modern.js",
    "@vaadin/router": "./libs/router.js"
  }
}
</script>

CSS Rules

• use CSS Grid for page-level layout with named grid areas
• use container queries (@container) over media queries for responsive design
• set container-type: inline-size on body
• use dvh units for viewport height (min-height: 100dvh)
• style custom elements directly by tag name (b-list { ... })
• use a lightweight CSS framework (Bulma) for utility classes — no Tailwind
• keep custom styles in a separate style.css
• use flexbox for component-level layout

Dependency Management

• third-party dependencies are installed in a separate libs/ directory (not the app root)
• Rollup bundles dependencies from node_modules as ESM into app/src/libs/
• import maps in index.html map bare specifiers to the bundled local files
• the application source never imports from node_modules — always through import maps

// rollup.config.js
export default [{
  input: [
    './node_modules/lit-html/lit-html.js',
    './node_modules/@vaadin/router/dist/router.js',
    './node_modules/@reduxjs/toolkit/dist/redux-toolkit.modern.mjs'
  ],
  output: { dir: "../app/src/libs", format: "esm" },
  plugins: [nodeResolve({ browser: true })]
}]

Testing

• E2E tests with Playwright in a separate tests/ directory
• test across Chromium, Firefox, and WebKit
• use role selectors (getByRole), label selectors (getByLabel), and visibility assertions
• use pressSequentially with delay for simulating realistic text input
• code coverage in a dedicated codecoverage/ directory using Monocart Coverage Reports
• tests verify user-visible behavior, not implementation details

Event Handling in Templates

• use lit-html event bindings: @click, @keyup, @input
• destructure event objects to extract relevant data: ({ target: { name, value } })
• use event.preventDefault() for form submissions
• validate forms using native HTML validation: form.reportValidity() and form.checkValidity()

What NOT to Do

• do not use Shadow DOM unless encapsulation is explicitly required
• do not use TypeScript — plain ES modules only
• do not install dependencies in the application root
• do not import from node_modules paths — use import maps
• do not add CSS preprocessors (Sass, Less, PostCSS)
• do not add bundlers for application code — only for third-party dependency packaging
• do not use React, Angular, Vue, Svelte, or any component framework
• do not use npm scripts for development workflow beyond npx vite
• do not create package.json in the application root
• do not dispatch Redux actions directly from boundary components — always go through control functions
• do not put business logic in boundary components — delegate to control layer

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