choonkending

const getUserFromLocalStorage = keyName => {
  const str = localStorage.getItem(keyName);
  try {
    return Right(JSON.parse(str));
  } catch (e) {
    return Left("Invalid or Corrupted User Data");
  }
};

choonkending

const transformCage = () => { /* ... */ };
const handleError = e => "Invalid or Corrupted User Data";
const handleSuccess = data => "Success";

const doSomethingWithUserData = () => 
  getUserFromLocalStorage('nickCage')
    .map(transformCage)
    .fold(handleError, handleSuccess);

choonkending

const transformCage = () => { /* ... */ };
const handleError = e => "failed to parse Cage";
const handleSuccess = data => "Success";
const ifEmpty = () => "Cage does not exist!";

const doSomethingWithUserData = () => 
  getUserFromLocalStorage('nickCage')
    .fold(handleError, 
      data => 
          data.map(transformCage)

choonkending

const getFeatures = data => {
  if (data && data.listing && data.listing.features) {
    const { bedrooms, bathrooms } = data.listing.features;
    const beds = bedrooms && bedrooms.value || 0;
    const baths = bathrooms && bathrooms.value || 0;
    if (beds === 0 && baths === 0) return null;
    return {
      bedrooms: beds,
      bathrooms: baths
    };

choonkending

/* Option types */
const Option = x => (x === undefined || x === null) ? None : Some(x);

const Some = x => ({
  filter: f => f(x) ? Some(x) : None,
  map: f => Some(f(x)),
  fold: (ifEmpty, f) => f(x),
  getOrElse: defaultValue => x
});

choonkending

const getUserFromLocalStorage = keyName => {
  const str = localStorage.getItem(keyName);
  try {
    return Right(Option(JSON.parse(str)));
  } catch (e) {
    return Left("Invalid or Corrupted User Data");
  }
};

choonkending

const getBedrooms = data => {
  const bedrooms = data.listing && data.listing.features && data.listing.features.bedrooms;
  if (bedrooms) {
      return bedrooms.value;
  }
  return 0;
};

choonkending

	map	flatMap	fold
Some(x)

choonkending

	Some(x)	None
map	map<V>(f: (value: U) => V): Some<V>

choonkending

Use Case

/* @flow */
type Props = { foo: string };
const View = ({ foo }: Props) => <div>{ foo }</div>;

// Consumer renders View