map, filter from reduce

reduce-all-the-things.js

function mapper(fn, arr) {
  return arr.reduce((acc, i) => [...acc, fn(i)], [])
}

function filterer(pred, arr) {
  return arr.reduce((acc, i) => pred(i) ? [...acc, i] : acc, [])
}

console.log(mapper( x => x ** 2, [1, 2, 3] ))   // => [1, 4, 9]
console.log(filterer( x => x >= 2, [1, 2, 3] )) // => [2, 3]