krzysztofczernek

// Example from https://github.com/loop-recur/FPJS-Class/blob/master/part1_exercises/exercises/compose/compose_exercises.js

// We can use ramda to avoid writing all helpers ourselves
const R = require('ramda')

const fastestCar = cars => {
  const sorted = R.sortBy(car => car.horsepower, cars)
  const fastest = R.last(sorted)
  return fastest.name
}

krzysztofczernek

const users = [
  {firstName: 'Jane', lastName: 'Doe'},
  {firstName: 'John', lastName: 'Doe'}
]

users.map(user => user.firstName) // ["Jane", "John"]

// Let's make this point-free. First:

const prop = propName => obj => obj[propName]

krzysztofczernek

['1', '12', '123'].map(num => parseInt(num)) // [1, 12, 123]

// This works nicely, however making it point-free is not as simple as:

['1', '12', '123'].map(parseInt) // [1, NaN, 1]

// This breaks because `map` passes more than just one argument to `parseInt` – it passes `currentValue`, `index`, and `array`
// `parseInt` then accepts the second argument as `radix`
// More information: 
// * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map

krzysztofczernek

const getUserDisplayName = user => `${user.firstName} ${user.lastName}`

const users = [
  {firstName: 'Jane', lastName: 'Doe'},
  {firstName: 'John', lastName: 'Doe'}
]

users.map(getUserDisplayName) // ["Jane Doe", "John Doe"]

krzysztofczernek

const getUserDisplayName = user => `${user.firstName} ${user.lastName}`

const users = [
  {firstName: 'Jane', lastName: 'Doe'},
  {firstName: 'John', lastName: 'Doe'}
]

users.map(user => getUserDisplayName(user)) // ["Jane Doe", "John Doe"]

krzysztofczernek

const map = fn => collection => collection.map(fn)

// Alternatively, import map from ramda:
// import { map } from 'ramda'

const doSmth = number => { 
  return String(number * 2)
}

console.log(doSmth(10)) // "20"

krzysztofczernek

// Traditional way, no composition
const activeProjectsDetails1 = projects => {
  const active = projects.filter(project => project.isActive)
  const detailsOnly = projects.map(project => pickProjectDetails(project))
  return orderBy(detailsOnly, ['updatedAt'])
}

// Composition
const activeProjectsDetails2 = projects => pipe(
  filter(isActive),

krzysztofczernek

// Traditional way, no composition
const calculate1 = x => (x + 1) * 2

// Composition
const calculate2 = compose(multiplyBy(2), add(1))

krzysztofczernek

const double = x => x * 2
const getLength = string => string.length

const map = fn => collection => collection.map(fn)

const messages = ['Hello', 'Bye']

const pipe = (...fns) => input => fns.reduce((mem, fn) => fn(mem), input)

const doubleLengths = pipe(map(getLength), map(double))(messages)

krzysztofczernek

const double = x => x * 2
const getLength = string => string.length

const map = (collection, fn) => collection.map(fn)

const messages = ['Hello', 'Bye']

const mapToLengths = collection => map(collection, getLength)
const mapToDoubles = collection => map(collection, double)
const pipe = (...fns) => input => fns.reduce((mem, fn) => fn(mem), input)