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

functional3.js

const Right = x =>
  ({
    x,
    map: f => Right(f(x)),
    inspect: _ => `Right(${x})`,
    isLeft: _ => false,
    chain: f => f(x),
  })

const Left = x =>
  ({
    x,
    map: f => Left(x),
    inspect: _ => `Left(${x})`,
    isLeft: _ => true,
    chain: f => Left(x)
  })

const tryCatch = f => {
  try {
    return Right(f())
  } catch (e) {
    return Left('could not parse')
  }
}

const fromNullable = (error, a) => (a === undefined || a === null) ? Left(error) : Right(a)