arrow-functions.jsx

return (
  <div>
    <h2>Friends</h2>
    <ul>
      {this.state.friends.map(friend =>
        <Friend
          key={ friend.id }
          friend={ friend }
          isSelected={ this.isSelected( friend ) }
          toggleSelection={ this.toggleSelection }
        />
      )}
    </ul>
  </div>
);