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HTML

              
                <!--
Reactive component example, as described on https://component.kitchen/blog/posts/a-compact-javascript-mixin-for-creating-native-web-components-in-frpreact-style
-->

<template id="template">
  <button id="decrement">-</button>
  <span id="value"></span>
  <button id="increment">+</button>
</template>

<increment-decrement value="10"></increment-decrement>

              
            
!

CSS

              
                
              
            
!

JS

              
                import ReactiveMixin from 'https://unpkg.com/elix@3.4.1/src/ReactiveMixin.js';
import * as symbols from 'https://unpkg.com/elix@3.4.1/src/symbols.js';

// Create a native web component with reactive behavior.
class IncrementDecrement extends ReactiveMixin(HTMLElement) {

  // Allow the value property to be set via an attribute.
  attributeChangedCallback(attributeName, oldValue, newValue) {
    if (attributeName === 'value') {
      this.value = parseInt(newValue);
    }
  }

  // This property becomes the value of this.state at constructor time.
  get defaultState() {
    return { value: 0 };
  }

  // Expose "value" as an attribute.
  static get observedAttributes() {
    return ['value'];
  }

  // Provide a public property that gets/sets state.
  get value() {
    return this.state.value;
  }
  set value(value) {
    this.setState({ value });
  }

  // Render the current state to the DOM.
  //
  // ReactiveMixin invokes this method when the state changes. This method
  // would typically invoke a rendering engine (virtual-dom, lit-html,
  // etc.).
  //
  // In this example, we'll just populate the DOM with an HTML template,
  // then update the DOM directly with DOM API calls. This actually isn't
  // so bad, and is the fastest possible way to render state to the DOM.
  [symbols.render]() {
    if (!this.shadowRoot) {
      // First time we render, create the shadow subtree.
      const root = this.attachShadow({ mode: 'open' });
      const template = document.querySelector('#template');
      const clone = document.importNode(template.content, true);
      root.appendChild(clone);
      // Wire up event handlers.
      root.querySelector('#decrement').addEventListener('click', () => {
        this.value--;
      });
      root.querySelector('#increment').addEventListener('click', () => {
        this.value++;
      });
    }
    // Render the state into the shadow.
    this.shadowRoot.querySelector('#value').textContent = this.state.value;
  }

}

customElements.define('increment-decrement', IncrementDecrement);
              
            
!
999px

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