I (roughly) defined (almost) every array method using recursion 😂

So... I have decided to define every array methods using recursion. (I haven't really tested all of them... so there might be some errors.)

Also, I only defined the "essence" of most methods. Didn't follow the complete spec for most.

Why?

Why not?

How is this useful?

It is not.

Array.from

Array.from takes in two kinds of objects.

  1. Array-like objects that have a length property with zero-indexed elements
  2. Iterable objects that have an iterator at [Symbol.iterator]
const arrayFrom = (o) => {
  if ('length' in o) return arrayFromArrayLike(o)
  if (Symbol.iterator in o) return arrayFromIterator(o[Symbol.iterator]())
  return []
}

const arrayFromArrayLike = (arrayLikeObject) => {
  if (arrayLikeObject.length <= 0) return []
  return [
    ...arrayFromArrayLike({
      ...arrayLikeObject,
      length: arrayLikeObject.length - 1,
    }),
    arrayLikeObject[arrayLikeObject.length - 1],
  ]
}

const arrayFromIterator = (iterator) => {
  const { value, done } = iterator.next()
  if (done) return []
  return [value, ...arrayFromIterator(iterator)]
}

Note: we ignore the 2nd and 3rd arguments of Array.from. (see docs)

Array.of

const arrayOf = (...xs) => {
  if (xs.length <= 0) return []
  const [head, ...tail] = xs
  return [head, ...arrayOf(...tail)]
}

Array.prototype.concat

const concat = (xs, ...arrays) => {
  if (arrays.length <= 0) return xs
  const [ys, ...restArrays] = arrays
  if (ys.length <= 0) return concat(xs, ...restArrays)
  const [head, ...tail] = ys
  return concat([...xs, head], tail, ...restArrays)
}

Note: assuming concat only takes in 2 parameters

Array.prototype.entries

function* entries(xs, i = 0) {
  if (xs.length <= 0) return
  const [head, ...tail] = xs
  yield [i, head]
  yield* entries(tail, i + 1)
}

note: i does not exist in Array.prototype.entries

Array.prototype.every

const every = (xs, predicate) => {
  if (xs.length <= 0) return true
  const [head, ...tail] = xs
  return predicate(head) && every(tail, predicate)
}

Array.prototype.fill

const fill = (xs, k, start = 0, end = xs.length + 1) => {
  if (xs.length <= 0) return []
  const [head, ...tail] = xs
  if (start > 0) return [head, ...fill(tail, k, start - 1, end - 1)]
  return fillFromStart([head, ...tail], k, end)
}

const fillFromStart = (xs, k, end = xs.length + 1) => {
  if (xs.length <= 0) return []
  if (end <= 0) return xs
  const [_, ...tail] = xs
  return [k, ...fillFromStart(tail, k, end - 1)]
}

Array.prototype.filter

const filter = (xs, predicate) => {
  if (xs.length <= 0) return []
  const [head, ...tail] = xs
  return [
    ...(predicate(head) ? [head] : []),
    ...filter(tail, predicate)
  ]
}

Array.prototype.find

const find = (xs, predicate) => {
  if (xs.length <= 0) return undefined
  const [head, ...tail] = xs
  if (predicate(head)) return head
  return find(tail, predicate)
}

Array.prototype.findIndex

const findIndex - (xs, predicate) => {
  if (xs.length <= 0) return -1
  const [head, ...tail] = xs
  if (predicate(head)) return 0
  return findIndex(tail, predicate) + 1
}

Array.prototype.forEach

const forEach = (xs, fn) => {
  if (xs.length <= 0) return
  const [head, ...tail] = xs
  fn(head)
  forEach(tail, fn)
}

notes: ignoring index

Array.prototype.includes

const includes = (xs, predicate) => {
  if (xs.length <= 0) return false
  const [head, ...tail] = xs
  const predicate(head) || includes(tail, predicate)
}

Array.prototype.indexOf

const indexOf = (xs, x) => {
  if (xs.length <= 0) return -1
  const [head, ...tail] = xs
  if (head === x) return 0
  return indexOf(tail, x) + 1
}

Array.prototype.join

const join = (xs, separator = ',') => {
  if (xs.length <= 0) return ''
  const [head, ...tail] = xs
  return `${head}${separator}${join(tail, separator)}`
}

Array.prototype.map

const map = (xs, fn) => {
  if (xs.length <= 0) return []
  const [head, ...tail] = xs
  return [fn(head), ...map(tail, fn)]
}

Array.prototype.reduce

const reduce = (xs, fn, acc) => {
  if (xs.length <= 0) {
    if (typeof acc === 'undefined') {
      throw new TypeError('Reduce of empty array with no initial value')
    } else {
      return acc
    }
  }

  const [head, ...tail] = xs
  if (typeof acc === 'undefined') return reduce(tail, fn, head)
  return reduce(tail, fn, fn(acc, head))
}

Array.prototype.reverse

const reverse = (xs) => {
  if (xs.length <= 0) return []
  const [head, ...tail] = xs
  return [...reverse(xs), head]
}

Array.prototype.slice

Slice is a surprisingly annoying one to define. It needs to deal with negative indices, but you can't simply "mod" the numbers...

const slice = (xs, start = 0, end = xs.length) => {
  if (xs.length <= 0) return []
  if (start < 0) return slice(xs, Math.max(0, start + xs.length), end)
  if (end < 0) return slice(xs, start, Math.max(0, end + xs.length))
  const [head, ...tail] = xs

  if (end <= start) return []
  if (start > 0) return slice(tail, start - 1, end - 1)

  return [head, ...slice(tail, 0, end - 1)]
}

Array.prototype.some

const some = (xs, predicate) => {
  if (xs.length <= 0) return false
  const [head, ...tail] = xs
  return predicate(head) || some(tail, predicate)
}