React: Use Array.filter() to Dynamically Filter an Array and Use Map - FreeCodeCamp

ReactArrayFilterandMap.js

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      users: [
        {
          username: 'Jeff',
          online: true
        },
        {
          username: 'Alan',
          online: false
        },
        {
          username: 'Mary',
          online: true
        },
        {
          username: 'Jim',
          online: false
        },
        {
          username: 'Sara',
          online: true
        },
        {
          username: 'Laura',
          online: true
        }
      ]
    }
  }
  render() {
    const usersOnline = this.state.users.filter(user => user.online); 
    const renderOnline = usersOnline.map(user => <li key={user.username}>{user.username}</li>); 
    return (
       <div>
         <h1>Current Online Users: {usersOnline.length}</h1>
         <ul>
           {renderOnline}
         </ul>
       </div>
    );
  }
};