css transition styled components

import React, { Component } from "react";
import ReactDOM from "react-dom";
import { CSSTransition } from "react-transition-group";
import styled from "styled-components";

const Box = styled.div`
  width: 300px;
  height: 300px;
  background: red;
  transition: opacity 0.3s;

  // enter from
  &.fade-enter {
    opacity: 0;
  }

  // enter to
  &.fade-enter-active {
    opacity: 1;
  }

  // exit from
  &.fade-exit {
    opacity: 1;
  }

  // exit to 
  &.fade-exit-active {
    opacity: 0;
  }
}`;

export default class App extends Component {
  constructor() {
    super();
    this.state = {
      active: true
    };

    setInterval(() => this.setState({ active: !this.state.active }), 1000);
  }

  render() {
    return (
      <CSSTransition
        in={this.state.active}
        classNames="fade"
        timeout={300}
        unmountOnExit
      >
        {() => <Box />}
      </CSSTransition>
    );
  }
}