functools.js

function reduce(transform, append, arr, init) {
  const l = arr.length
  let i = 0
  let accum = init
  if (typeof accum === 'undefined') {
    accum = arr[0]
    i = 1
  }
  for (; i < l; i++) {
    accum = append(accum, transform(arr[i]))
  }
  return accum
}

function map(fn, arr) {
  return reduce(fn, (newArr, x) => {
    newArr.push(x)
    return newArr
  }, arr)
}

function filter(fn, arr) {
  return reduce(id, (newArr, x) => {
    if (fn(x)) {
      newArr.push(x)
    }
    return newArr
  })
}




function id(x) {
  return x
}