old_contents: import React, { Component } from 'react';
  import { Link } from 'react-router-dom'
  
  export default class Home extends Component {
    render() {
      return (
        <Link to='/app'>Join</Link>
      );
    }
  }
new_contents: import React, { Component } from 'react';
  import { Link } from 'react-router-dom'
  import App from './App';
  
  export default class Home extends Component {
    constructor(props) {
      super(props);
  
      this.state = {
        showSession: false
      };
  
      this.displaySession = this.displaySession.bind(this);
    }
  
    displaySession() {
      this.setState({
        showSession: true
      });
    }
  
    render() {
      return (
        <div>
          <button onClick={this.displaySession}>Join</button>
          {
            this.state.showSession ? <App /> : null
          }
        </div>
      );
    }
  }
subject: update home button