createPortal

.overlay {
  position: fixed;
  top: 50%;
  left: 50%;
  width: 260px;
  line-height: 200px;
  height: 200px;
  margin-left: -130px;
  margin-top: -100px;
  background-color: #fff;
  text-align: center;
  outline: 9999px solid rgba(0, 0, 0, 0.5);
}

.overlay div {
  display: inline;
}

<!doctype html>

<html>
  <head>
    <script src="https://unpkg.com/react@16.0.0/umd/react.development.js"></script>
    <script src="https://unpkg.com/react-dom@16.0.0/umd/react-dom.development.js"></script>
    <script src="https://unpkg.com/babel-standalone@6.26.0/babel.min.js"></script>
    <link rel="stylesheet" href="index.css">
    
  </head>

  <body>
    <div id="root"></div>
    <div id="overlay-root">
      
    </div>
    <script type="text/jsx" src="script.jsx"></script>
  </body>
</html>

//import React, { Component } from 'react';
//import ReactDOM from 'react-dom';
//import './index.css';

class Overlay extends React.Component {
  constructor(props) {
    super(props);
    this.overlayContainer = document.getElementById('overlay-root');
  }
   
  componentWillUnmount() {
    document.body.removeChild(this.overlayContainer);
  }

  render() {
    return ReactDOM.createPortal(
      <div className="overlay">
        <span onClick={this.props.onClose}>x</span>
        {this.props.children}
      </div>,
      this.overlayContainer
    );
  }
}

class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = { overlayActive: true };
  }

  closeOverlay = () => {
    this.setState({ overlayActive: false });
  };

  render() {
    return (
      <div>
        <h1>Dashboard</h1>
        {this.state.overlayActive &&
          <Overlay onClose={this.closeOverlay}>
            <div>Welcome</div>
          </Overlay>}
      </div>
    );
  }
}

ReactDOM.render(<App />, document.getElementById('root'));