Portals

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

ReactDOM.createPortal(child, container);

The first argument (child) is any renderable React child, such as an element, string, or fragment. The second argument (container) is a DOM element.

Usage

Normally, when you return an element from a component's render method, it's mounted into the DOM as a child of the nearest parent node:

render() {
  // React mounts a new div and renders the children into it
  return (
    <div>
      {this.props.children}
    </div>
  );
}

However, sometimes it's useful to insert a child into a different location in the DOM:

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
  );
}

A typical use case for portals 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. For example, dialogs, hovercards, and tooltips.

Note:
When working with portals, remember that managing keyboard focus becomes very important.
For modal dialogs, ensure that everyone can interact with them by following the WAI-ARIA Modal Authoring Practices.

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Event Bubbling Through Portals

Even though a portal can be anywhere in the DOM tree, it behaves like a normal React child in every other way. Features like context work exactly the same regardless of whether the child is a portal, as the portal still exists in the React tree regardless of position in the DOM tree.

This includes 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. Assuming the following HTML structure:

<html>
  <body>
    <div id="app-root"></div>
    <div id="modal-root"></div>
  </body>
</html>

A Parent component in #app-root would be able to catch an uncaught, bubbling event from the sibling node #modal-root.

// 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);

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Catching an event bubbling up from a portal in a parent component allows the development of more flexible abstractions that are not inherently reliant on portals. For example, if you render a <Modal /> component, the parent can capture its events regardless of whether it's implemented using portals.

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Last updated 3 years ago

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