From <https://gist.github.com/granolocks/cc4c5e0302fa5b5d9757>

caesar.js

const A = 'A'.charCodeAt(0)

const cipher = offset => character =>
  String.fromCharCode(A + (offset + character.toUpperCase().charCodeAt(0) - A) % 26)

const encode = (s, d) => s.replace(/\W/g,'').split('').map(cipher(d)).join('')
const decode = (s, d) => s.split('').map(cipher(26 - d)).join('')

decode(encode("Friends, Romans, Countrymen, lend me your ears!", 10), 10)
// => 'FRIENDSROMANSCOUNTRYMENLENDMEYOUREARS'

change.js

const COINS = [ 25, 10, 5, 1 ]

const assoc (o, k, v) => Object.assign({}, o, { [k]: v })

const change = val =>
  // Here we're reducing over two values to retain referential transparency.
  COINS.reduce(([result, pennies], coin) => (
    [ assoc(result, coin, Math.floor(pennies/coin)),
      pennies % coin ]),
    [{}, (val - Math.floor(val)) * 100])[0]

change(0.83)
// => { 25: 3, 10: 0, 5: 1, 1: 3 }

invert-index.js

const invertIndex = files => 
  files.reduce((textIndex, file, i) =>
    file.split(/\w+/g).reduce((textIndex, word) => {
      if(!textIndex[word]) textIndex[word] = new Set()
      textIndex[word].add(i)

      return textIndex
    }, textIndex))