JS pattern matching

patternmatching.js

const Pattern  = testers => transformers => val => {
  const match = Object.keys(testers).find(k => testers[k](val))
  return transformers[match](val)
}

const Num = Pattern({
  Odd: n => n % 2,
  Even: n => !(n % 2)
})

const numberToString = Num({
 Odd: x => '+',
 Even: x => '-'
})

;[4,8,15,16,23,42].map(numberToString).join('') //=> "--+-+-"