Having planned to write a dialog using React for a long time, now, it's the time to fullfill the promise. As a matter of fact, it's pretty easy.

The key is the use of React interface React.createPortal(element, domContainer), which inserts the mounted node of element into domContainer(ofter document.body) only once.

So, we can write a dialog or modal box like this:

function Dialog() {
    return React.createPortal( <div>Dialog contents</div>, document.body )
}

A new 'div' will appear inside 'body' :

A complete DEMO:

class Modal extends React.Component {
  render() {
    const {
      visible,
      onClose
    } = this.props
    return visible && ReactDOM.createPortal(<StyledModalRoot>
      <div className="box">
        Content
        <br/>
        <button onClick={onClose}>Close</button>
      </div>
    </StyledModalRoot>, document.body)
  }
}

class App extends React.Component {
  state = {
    visibleModal: false
  }
  showModal = () => this.setState( { visibleModal: true } )
  handleCloseModal = () => this.setState( { visibleModal: false } )
  render() {
    const { visibleModal } = this.state
    return <div style={{padding: '20px'}}>
    <button onClick={ this.showModal }>Show Modal</button>
    <Modal visible={visibleModal} onClose={ this.handleCloseModal } />
  </div>
  }
}

const StyledModalRoot = styled.div`
  position: fixed;
  z-index: 1001;
  left: 0;
  top: 0;
  display: grid;
  place-items: center;
  width: 100%;
  height: 100%;
  background: rgba( 0, 0, 0, 0.2 );

  >.box {
    position: relative;
    display: grid;
    place-items: center;
    width: 80%;
    height: 80%;
    background: white;
    border-radius: 10px;
    box-shadow: 0px 3px 5px -1px rgba(0,0,0,0.2), 0px 5px 8px 0px rgba(0,0,0,0.14), 0px 1px 14px 0px rgba(0,0,0,0.12);
  }
`