Pen Settings

HTML

CSS

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

JavaScript

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

Packages

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.

Behavior

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.

HTML

              
                
<div id="root"></div>
              
            
!

CSS

              
                
              
            
!

JS

              
                let React = (function() {
  let createElement = (type, options, ...children) => {
    return {
      type,
      props: Object.assign(
        {
          children: children.map(child =>
            typeof child == "object"
            ? child
            : { type: "TEXT_NODE", props: { value: child } }
          )
        },
        options
      )
    };
  };

  let isAttribute = attrName => attrName != "children" && !isEvent(attrName);
  let isEvent = attrName => attrName.startsWith("on");
  let eventName = event => event.substring(2).toLowerCase();

  let createInstance = (type, props) => {
    if (type == "TEXT_NODE") {
      return document.createTextNode(props.value);
    }

    if (typeof type == 'function') {
      let instance = new type(props);
      let childElement = instance.render();
      let childNode = createInstance(childElement.type, childElement.props);
      instance.dom = childNode;
      return childNode;
    } 

    let node = document.createElement(type);


    for (let key in props) {
      if (isEvent(key)) 
        node.addEventListener(eventName(key), props[key]);
      else if (isAttribute(key)) 
        node.setAttribute(key, props[key]);
    }

    // loop through children and recursivelly render them
    let children = props.children || [];
    children.forEach(child => {
      let childNode = createInstance(child.type, child.props);
      // then append to it's parent
      node.appendChild(childNode);
    });
    return node;
  };

  let render = (element, parentNode) => {
    let node = createInstance(element.type, element.props);
    parentNode.appendChild(node);
  };

  class Component {
    constructor(props) {
      this.props = props;
      this.state = {};
    }

    setState(partialState) {
      this.state = Object.assign({}, this.state, partialState);
      const element = this.render();
      const node = createInstance(element.type, element.props);
      const parent = this.dom.parentNode;
      parent.replaceChild(node, this.dom)
      this.dom = node;
    }
  }

  return { createElement, render, Component };
})();

class Favorite extends React.Component {
  constructor(props) {
    super(props);
    this.state = { currentValue: 1 };
  }
  
  render(){
    return <div>
      <span>{ this.state.currentValue }</span>
      <button onClick={(evt) => this.setState({currentValue: this.state.currentValue + 1})} >Like</button>
      <button onClick={(evt) => this.state.currentValue > 0 && this.setState({currentValue: this.state.currentValue - 1})} >Unlike</button>
    </div>
  }
}
    
React.render(<div><Favorite/><Favorite/></div>, document.getElementById("root"));

              
            
!
999px

Console