index.js

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

class App extends Component {
  state = {
    display: false,
  };

  toggle = () => {
    this.setState(prevstate => ({
      display: !prevstate.display,
    }));
  };
  render() {
    return (
      <div className="container">
        <button 
          className={cx('toggler', {
            'toggler--active': this.state.display,
          })}
          onClick={this.toggle}>Show</button>
        {this.state.display && (
          <div className="menu">
            <ul className="list">
              <li className="list-item">Rajat</li>
              <li className="list-item">Writes Posts</li>
              <li className="list-item">Loves Pizza</li>
            </ul>
          </div>
        )}
      </div>
    )
  }
}

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