Pen Settings



CSS Base

Vendor Prefixing

Add External Stylesheets/Pens

Any URL's added here will be added as <link>s in order, and before the CSS in the editor. You can use the CSS from another Pen by using it's URL and the proper URL extention.

+ add another resource


Babel includes JSX processing.

Add External Scripts/Pens

Any URL's added here will be added as <script>s in order, and run before the JavaScript in the editor. You can use the URL of any other Pen and it will include the JavaScript from that Pen.

+ add another resource


Add Packages

Search for and use JavaScript packages from npm here. By selecting a package, an import statement will be added to the top of the JavaScript editor for this package.


Save Automatically?

If active, Pens will autosave every 30 seconds after being saved once.

Auto-Updating Preview

If enabled, the preview panel updates automatically as you code. If disabled, use the "Run" button to update.

Format on Save

If enabled, your code will be formatted when you actively save your Pen. Note: your code becomes un-folded during formatting.

Editor Settings

Code Indentation

Want to change your Syntax Highlighting theme, Fonts and more?

Visit your global Editor Settings.


Reactive component example, as described on

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

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





                import ReactiveMixin from '';
import * as symbols from '';

// 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);
      // Wire up event handlers.
      root.querySelector('#decrement').addEventListener('click', () => {
      root.querySelector('#increment').addEventListener('click', () => {
    // Render the state into the shadow.
    this.shadowRoot.querySelector('#value').textContent = this.state.value;


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