index.js

function groupByFirstLetter(wordCollection) {
  return wordCollection.reduce((result, word) => {
    const firstLetter = word.charAt(0)
    if (!(firstLetter in result)) {
      result[firstLetter] = []
    }
    result[firstLetter].push(word)
    return result
  }, {})
}

function capitalizeFirstLetter(string) {
  return string.charAt(0).toUpperCase() + string.slice(1)
}

function pickRandomly(wordCollection) {
  return wordCollection[Math.floor(Math.random() * wordCollection.length)]
}

function findCommonLetters(lettersA, lettersB) {
  return lettersA.reduce((result, letter) => {
    if (lettersB.indexOf(letter) > -1) {
      result.push(letter)
    }
    return result
  }, [])
}

const animals = groupByFirstLetter(require('./animals.json'))
const adjectives = groupByFirstLetter(require('./adjectives.json'))

const possibleLetters = findCommonLetters(
  Object.keys(adjectives),
  Object.keys(animals)
)

function findRandomAdjective(letter) {
  return pickRandomly(adjectives[letter])
}

function findRandomAnimalName(letter) {
  return pickRandomly(animals[letter]).split(' ').join('-')
}

function generateRandomAnimalName() {
  const letter = pickRandomly(possibleLetters)
  const adjective = findRandomAdjective(letter)
  const animal = findRandomAnimalName(letter)
  return `${capitalizeFirstLetter(adjective)} ${animal}`
}

if (require.main === module) {
  console.log(generateRandomAnimalName())
}

module.exports = generateRandomAnimalName