https://medium.com/@riccardoodone/fp-in-vanilla-javascript-a-rookies-intro-e32ad13484f4

functional_explanation2.js

// tryCatch :: Function => Either(String)
const tryCatch = f => {
  try {
    return Right(f())
  } catch (e) {
    return Left('could not parse')
  }
}

// parse :: String -> Either [String]
const parse = s => tryCatch(() => JSON.parse(s))

// fromNullable :: String -> a -> Either(a)
const fromNullable = error => a =>
  (a === undefined || a === null) ? Left(error) : Right(a)

// first :: [a] -> Either(a)
const first = xs => fromNullable('empty array')(xs[0])

// format :: String => Either(String)
const format = s => Right(s).map(upper).map(shout)

// functional :: String -> Either(String)
const functional = s => Right(s).chain(parse).chain(first).chain(format)