index.js

class App extends React.Component {
    constructor() {
      super();

      this.state = { showPicker: true };
    }
    
    componentDidUpdate() {
      console.log(document.querySelectorAll(".pika-single"));
    }

    onDateChange(date) {
      this.setState({ date });
    }
    
    toggleShowPicker() {
      this.setState({ showPicker: !this.state.showPicker });
    }

    render() {
      const { date } = this.state;

      return (
        <div className="px3">
          <h1>React Pikaday Component</h1>
          
          <div>
            <button onClick={this.toggleShowPicker.bind(this)}>
              Toggle
            </button>
          </div>
          
          {this.state.showPicker &&
            <ReactPikadayComponent
              className="field mb2"
              placeholder="Select Date"
              format="YYYY/MM/DD"
              maxDate={new Date('2020-07-26')}
              theme="dark-theme"
              value={new Date(date)}
              onChange={::this.onDateChange}
            />
          }
          
          {date ? (
            <div>Selected date: {new Date(date).toString()}</div>
          ) : (
            <div>There is no selected date!</div>
          )}
        </div>
      );
    }
}

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