Kleisli composition example (JSON parsing + deep property)

index.js

// following along https://medium.com/@luijar/kliesli-compositions-in-javascript-7e1a7218f0c4
const R = require('ramda')
const Maybe = require('ramda-fantasy').Maybe

// parse JSON string into object
// parse :: String -> Object | Null
function parse(s) {
  try {
    return JSON.parse(s)
  } catch (e) {
    return null
  }
}

// prop :: (Object, String) -> Object | String | Null
const prop = (obj, name) => obj[name]

// maybeParseJson :: String -> Maybe
const maybeParseJson = json => Maybe.toMaybe(parse(json))

// maybeProp :: String -> Object -> Maybe
const maybeProp = prop => obj => Maybe.toMaybe(obj[prop])

// convert a "simple" function to return Maybe
// https://github.com/ramda/ramda-fantasy/blob/master/docs/Maybe.md#maybeof
const maybeUpper = R.compose(Maybe.of, R.toUpper)

// a typical user object is
/*
{
  user:
   {
     address:
       {
         state: "fl"
       }
    }
}
*/
// getStateCode :: String -> Maybe
const getStateCode = R.composeK(
  maybeUpper,
  maybeProp('state'),
  maybeProp('address'),
  maybeProp('user'),
  maybeParseJson
)

module.exports = getStateCode

spec.js

// testing getStateCode
const getStateCode = require('./index')

it('extracts from valid string', () => {
  const s = '{"user": {"address": {"state": "in"}}}'
  const state = getStateCode(s).getOrElse('an error')
  console.assert(state === 'IN', state)
})

it('handles invalid JSON string', () => {
  // notice missing quotes
  const s = '{"user": {"address": {state: in}}}'
  const state = getStateCode(s).getOrElse('an error')
  console.assert(state === 'an error', state)
})

it('handles missing property', () => {
  // notice missing address
  const s = '{"user": {}}'
  const state = getStateCode(s).getOrElse('an error')
  console.assert(state === 'an error', state)
})