// imperative, state mutation, no separation of side effects
(function latinize(phrase){
    let phraseArray = phrase.split(' ');

    const reducer = phraseArray.reduce((accumulator, currentValue) => {
        let firstLetter = currentValue.substring(0,1);
        let rest = currentValue.substring(1);
        let latin = 'ay ';

        accumulator += rest + firstLetter + latin;

        return accumulator
    }, (''))

    console.log(reducer)


})('quick brown fox')

// declarative, no state mutation, separation of side effects
// both my examples do the same thing

const latinize = phrase => (
  phrase.split(' ').reduce((newPhrase, word) => {
    const [ first, ...rest ] = word;
    return `${newPhrase} ${rest.join('')}${first}ay`
  }, '').trim()
)

console.log(latinize('quick brown fox'))

const latinize = phrase => (
  phrase.split(' ').reduce((newPhrase, word, i) => {
    const [ first, ...rest ] = word
    return i === 0
      ? `${rest.join('')}${first}ay`
      : `${newPhrase} ${rest.join('')}${first}ay`
  }, '')
)

console.log(latinize('quick brown fox'))

// another version of your solution that incorporates some of what I did
// that might be more understandable to you

(function latinize(phrase) {
  const latin = "ay ";

  const newPhrase = phrase.split(" ").reduce((accumulator, currentValue) => {
    const firstLetter = currentValue.substring(0, 1);
    const rest = currentValue.substring(1);

    return accumulator + rest + firstLetter + latin;
  }, "");

  console.log(newPhrase);
})("quick brown fox");