Pen Settings



CSS Base

Vendor Prefixing

Add External Stylesheets/Pens

Any URLs 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 its URL and the proper URL extension.

+ 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.


Auto Save

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.


                <div id="counter"></div>

<button id="increment">Increment</button>




                // Creating a simple object that holds a state. Will be used as the target object.
const data = {
  counter: 0

// Configure the handler object and how we are going to follow changes on state and update the DOM.
const handler = {
  get(target, property) {
    // Get the value of a specific property
    return target[property];
  set(target, property, value) {
    // Set the value of a specific property
    target[property] = value;

    // Update the DOM

// Creating the Proxy
const reactiveData = new Proxy(data, handler);

// Listen to click event to set the new counter value
const incrementButton = document.getElementById('increment');
if (incrementButton) {
  incrementButton.addEventListener('click', () => {

// Function for updating the DOM
function updateView() {
  const counterElement = document.getElementById('counter');
  if (counterElement) {
    counterElement.textContent = reactiveData.counter;

// Initial DOM update