Professor Frisby's functional programming exercises (Part 2: compose)

fp.exercises_2.js

const accounting = require('accounting')
const expect = require('expect')
const {
  add,
  compose,
  concat,
  filter,
  flip,
  head,
  join,
  last,
  map,
  prop,
  reduce,
  replace,
  sortBy,
  toLower,
} = require('ramda')

// Example Data
const CARS = [
  {
    name: 'Ferrari FF',
    horsepower: 660,
    dollar_value: 700000,
    in_stock: true,
  },
  {
    name: 'Spyker C12 Zagato',
    horsepower: 650,
    dollar_value: 648000,
    in_stock: false,
  },
  {
    name: 'Jaguar XKR-S',
    horsepower: 550,
    dollar_value: 132000,
    in_stock: false,
  },
  {
    name: 'Audi R8',
    horsepower: 525,
    dollar_value: 114200,
    in_stock: false,
  },
  {
    name: 'Aston Martin One-77',
    horsepower: 750,
    dollar_value: 1850000,
    in_stock: true,
  },
  {
    name: 'Pagani Huayra',
    horsepower: 700,
    dollar_value: 1300000,
    in_stock: false,
  },
]

// Exercise 1:
// ============
// Use _.compose() to rewrite the function below. Hint: _.prop() is curried.
const isLastInStock = compose(prop('in_stock'), last)

expect(isLastInStock(CARS)).toBe(false)

// Exercise 2:
// ============
// Use _.compose(), _.prop() and _.head() to retrieve the name of the first car.
const nameOfFirstCar = compose(prop('name'), head)

expect(nameOfFirstCar(CARS)).toBe('Ferrari FF')

// Exercise 3:
// ============
// Use the helper function _average to refactor averageDollarValue as a composition.
const _average = xs => reduce(add, 0, xs) / xs.length // <- LEAVE BE

const averageDollarValue = compose(_average, map(prop('dollar_value')))

expect(averageDollarValue(CARS)).toBe(790700)

// Exercise 4:
// ============
// Write a function: sanitizeNames() using compose that returns a list of lowercase and underscored car's names:
// e.g: sanitizeNames([{name: 'Ferrari FF', horsepower: 660, dollar_value: 700000, in_stock: true}]) //=> ['ferrari_ff'].

const _underscore = replace(/\W+/g, '_') // <-- leave this alone and use to sanitize

const sanitizeNames = map(compose(_underscore, toLower, prop('name')))

const expected = [
  'ferrari_ff',
  'spyker_c12_zagato',
  'jaguar_xkr_s',
  'audi_r8',
  'aston_martin_one_77',
  'pagani_huayra',
]

expect(sanitizeNames(CARS)).toEqual(expected)

// Bonus 1:
// ============
// Refactor availablePrices with compose.

const _formatPrice = compose(accounting.formatMoney, prop('dollar_value'))
const availablePrices = compose(
  join(', '),
  map(_formatPrice),
  filter(prop('in_stock'))
)

expect(availablePrices(CARS)).toBe('$700,000.00, $1,850,000.00')

// Bonus 2:
// ============
// Refactor to pointfree. Hint: you can use _.flip().

const _append = flip(concat)
const fastestCar = compose(
  _append(' is the fastest'),
  prop('name'),
  last,
  sortBy(prop('horsepower'))
)

expect(fastestCar(CARS)).toBe('Aston Martin One-77 is the fastest')

console.info('You are now a genius of compose!!! 🤓 👏🏼 👏🏼 👏🏼')