perms.js

// here i'm thinking i needed to use `reduce` twice for some reason
const perms = (str) => {
  const words = str.split(' ')
  return words.reduce((p, _, i) => {
    return p.concat([words.reduce((s, word, j) => {
      return j >= i ? s + ` ${word}` : s
    }, '').trim()])
  }, [])
}

perms2.js

// realising the first fold has simply reproduced `map`
const perms2 = (str) => {
  const words = str.split(' ')
  return words.map((_, i) => {
    return words.reduce((s, word, j) => {
      return j >= i ? s + ` ${word}` : s
    }, '').trim()
  })
}

perms3.js

// and finally realising i didn't need to fold at all 
// and could simple `slice` the word array
const perms3 = (str) => {
  const words = str.split(' ')
  return words.map((_, i) => words.slice(i).join(' '))
}

perms4.js

// and seeing as js gives us the original array as the third argument to `map`
// we can simply even further
const perms4 = (str) => str.split(' ').map((_, i, words) => words.slice(i).join(' '))