Portals

Provide a first-class way to render children into a DOM node that exists outside the DOM hierarchy of the parent component.

A typical use case is when a parent component has an overflow: hidden or z-index style, but you need the child to visually “break out” of its container.

Demonstration code

render() {
  // React does *not* create a new div. It renders the children into `domNode`.
  // `domNode` is any valid DOM node, regardless of its location in the DOM.
  return ReactDOM.createPortal(
    this.props.children,
    domNode
  );
}

Event Bubbling

An event fired from inside a portal will propagate to ancestors in the containing React tree, even if those elements are not ancestors in the DOM tree.

Demonstration code

// These two containers are siblings in the DOM
const appRoot = document.getElementById('app-root');
const modalRoot = document.getElementById('modal-root');

class Modal extends React.Component {
  constructor(props) {
    super(props);
    this.el = document.createElement('div');
  }

  componentDidMount() {
    // The portal element is inserted in the DOM tree after
    // the Modal's children are mounted, meaning that children
    // will be mounted on a detached DOM node. If a child
    // component requires to be attached to the DOM tree
    // immediately when mounted, for example to measure a
    // DOM node, or uses 'autoFocus' in a descendant, add
    // state to Modal and only render the children when Modal
    // is inserted in the DOM tree.
    modalRoot.appendChild(this.el);
  }

  componentWillUnmount() {
    modalRoot.removeChild(this.el);
  }

  render() {
    return ReactDOM.createPortal(
      this.props.children,
      this.el,
    );
  }
}

class Parent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {clicks: 0};
    this.handleClick = this.handleClick.bind(this);
  }

  handleClick() {
    // This will fire when the button in Child is clicked,
    // updating Parent's state, even though button
    // is not direct descendant in the DOM.
    this.setState(state => ({
      clicks: state.clicks + 1
    }));
  }

  render() {
    return (
      <div onClick={this.handleClick}>
        <p>Number of clicks: {this.state.clicks}</p>
        <p>
          Open up the browser DevTools
          to observe that the button
          is not a child of the div
          with the onClick handler.
        </p>
        <Modal>
          <Child />
        </Modal>
      </div>
    );
  }
}

function Child() {
  // The click event on this button will bubble up to parent,
  // because there is no 'onClick' attribute defined
  return (
    <div className="modal">
      <button>Click</button>
    </div>
  );
}

ReactDOM.render(<Parent />, appRoot);

Related concepts

Portals — Structure map

Clickable & Draggable!

Portals — Related pages: