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="root"></div>




                import { useEffect, useState, useRef, useCallback } from "";
import ReactDOM from "";

function debounce(func, wait) {
	var timeout;
	return function() {
		var context = this, args = arguments;
		var later = function() {
			timeout = null;
			func.apply(context, args);
		var callNow = immediate && !timeout;
		timeout = setTimeout(later, wait);
		if (callNow) func.apply(context, args);

  config: { attributes: true, childList: true, subtree: true },
  debounceTime: 0
 * This custom hooks abstracts the usage of the Mutation Observer with React components.
 * Watch for changes being made to the DOM tree and trigger a custom callback.
 * @param {Element} targetEl DOM element to be observed
 * @param {Function} cb callback that will run when there's a change in targetEl or any
 * child element (depending on the provided options)
 * @param {Object} options
 * @param {Object} options.config check [options](
 * @param {number} [options.debounceTime=0] a number that represents the amount of time in ms
 * that you which to debounce the call to the provided callback function
function useMutationObservable(targetEl, cb, options = DEFAULT_OPTIONS) {
  const [observer, setObserver] = useState(null);

  useEffect(() => {
    if (!cb || typeof cb !== "function") {
        `You must provide a valida callback function, instead you've provided ${cb}`
    const { debounceTime } = options;
    const obs = new MutationObserver(
      debounceTime > 0 ? debounce(cb, debounceTime) : cb
  }, [cb, options, setObserver]);

  useEffect(() => {
    if (!observer) return;

    if (!targetEl) {
        `You must provide a valid DOM element to observe, instead you've provided ${targetEl}`

    const { config } = options;

    try {
      observer.observe(targetEl, config);
    } catch (e) {

    return () => {
      if (observer) {
  }, [observer, targetEl, options]);

function App() {
  const listRef = useRef();
  const [count, setCount] = useState(2);
  const [fruits, setFruits] = useState(["apple", "peach"]);
  const onListMutation = useCallback(
    (mutationList) => {
  useMutationObservable(listRef.current, onListMutation);

  return (
      <span>{`Added ${count} fruits`}</span>
      <br />
        onClick={() => setFruits([...fruits, `random fruit ${fruits.length}`])}
        Add random fruit
      <ul ref={listRef}>
        { => (
          <li key={f}>{f}</li>

const el = document.querySelector("#root");
ReactDOM.render(<App/>, el);