index.js

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

class App extends Component {
  heroes = [
    {
      name: "Batman",
      id: uuid(),
    },
    {
      name: "Wonder Woman",
      id: uuid(),
    },
    {
      name: "Titans",
      id: uuid(),
    },
    {
      name: "Supergirl",
      id: uuid(),
    },
    {
      name: "The Flash",
      id: uuid(),
    },
  ];
  state = {
    favComics: [],
  };

  toggleInFavComics = id => {
    let favComics;
    const isItemFavorite = this.state.favComics.find(
      favorite => favorite.id === id
    );
    if (isItemFavorite) {
      favComics = this.state.favComics.filter(
        favorite => favorite.id !== id
      );
    } else {
      favComics = [
        ...this.state.favComics,
        this.heroes.find(item => id === item.id),
      ]
    }
    this.setState({favComics});
  };
  render() {
    return (
      <div className="container">
        <ul className="comics">
          {this.heroes.map(({id, name}) => (
            <li 
              key={id}
              className="comic"
              onClick={() => this.toggleInFavComics(id)}
            >
              {name}
              <span className="star">
                {this.state.favComics.find(
                  favorite => favorite.id === id
                ) ? '★' : '☆'}
              </span>
            </li>
          ))}
        </ul>
        <div className="favorites">
          <p>Favorites:</p>
          {this.state.favComics.map(
            ({id, name}) => (
              <li className="favorite">{name}</li>
            )
          )}
        </div>
      </div>
    );
  }
}

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