Functional null-check and code-branching with React

either-react.jsx

/**
 * This is something I threw together as a quick test.
 * It's a contrived example because you'd likely just have one component for message and include something like: 
 * <p>{message.body || 'No message.'}</p>
 * 
 * However, when I'm writing components, I often have to handle different UI treatments depending on the data.
 * It's the same overall component, same logical chunk of UI, but it might take on three different forms, and
 * those differences may have drastically different markup. Those situations make it easy start writing a lot 
 * of imperative code, and I always feel dissatisfied when I have to include if statements in my render functions. 
 *
 * So, take a look at Messages, my first hack at a completely declarative bit of React.
 * Feedback, thoughts, and miscellany all welcome. You can find me at @deanboop on twitter.
 */

/*
 * Either type and fromNullable helper from Brian Lonsdorf's excellent new egghead.io series.
 * watch them here: https://egghead.io/lessons/javascript-linear-data-flow-with-container-style-types-box
 */

const Right = x => ({
  chain: f => f(x),
  map: f => Right(f(x)),
  fold: (f, g) => g(x),
  inspect: () => `Right(${x})`
});

const Left = (x) => ({
  chain: (f) => Left(x),
  map: (f) => Left(x),
  fold: (f, g) => f(x),
  inspect: () => `Left(${x})`
});

const fromNullable = x =>
  x != null ? Right(x) : Left(null);

const messages = [
  { subject: 'Your guitar', body: 'You forgot it at my house', sentBy: 'joy@foo.com' },
  { subject: 'Call me when you can', body: null, sentBy: 'frederico@bar.com' },
  { subject: 'Will meet you @5 eom', body: '', sentBy: 'paul@boz.com' },
  { subject: 'Hey!', body: 'Let\'s grab lunch this week. When are you free?', sentBy: 'akhila@baz.com' }
];

const ShortMessage = ({ message }) => (
  <div className="message">
    <h2>{message.subject}</h2>
    <p><i>No message body.</i></p>
    <p>from {message.sentBy}</p>
  </div>
);

const LongMessage = ({ message }) => (
  <div className="message">
    <h2>{message.subject}</h2>
    <p>{message.body}</p>
    <p>{message.sentBy}</p>
  </div>
);

// This is the bit to pay attention to. 
const Message =({ message }) => fromNullable(message.body)
  .fold(() => <ShortMessage message={message} />,
        () => <LongMessage message={message} />);

const App = ({ messages }) => (
  <main>
    <h1>messages</h1>
    <section>
      {messages.map((m, i) => <Message key={i} message={m} />)}
    </section>
  </main>
);

ReactDOM.render(<App messages={messages} />, document.getElementById('root'));