derived collection methods

derived collection methods.js

// given: Array.prototype.forEach, derive:
// 1. reduce
// 2. concat
// 3. map
// 4. filter
// 5. flatten
// 6. flatMap

const reduce = (xs, f, z) => {
  var acc = z
  xs.forEach(x => acc = f(acc, x))
  return acc
}

const concat = (xs, ys) => {
  const append = (xs, y) => {
    xs.push(y)
    return xs
  }
  return reduce(ys, append, xs)
}

const map = (xs, f) => xs.reduce((a, b) => a.concat(f(b)), [])

const filter = (xs, p) => xs.reduce((a, b) => p(b) ? a.concat(b) : a, [])

const flatten = (xs) => xs.reduce((a, b) => a.concat(b), [])

const flatMap = (xs, f) => xs.reduce((a, b) => flatten(a.concat(f(b))), [])

Object.defineProperty(Array.prototype, 'my', {
  get() {
    return {
      reduce: (f, z) => reduce(this, f, z),
      concat: (xs) => concat(this, xs),
      map: (f) => map(this, f),
      filter: (f) => filter(this, f),
      flatten: () => flatten(this),
      flatMap: (f) => flatMap(this, f)
    }
  }
})

/* -- examples below -- */

console.log(
  [1, 2, 3].my.reduce((a, b) => a + b, 0)
)

console.log(
  [1, 2, 3].my.concat([4, 5])
)

console.log(
  [1, 2, 3].my.map(x => x + 1)
)

console.log(
  [1, 2, 3, 4, 5, 6, 7].my.filter(x => x % 2 === 0)
)

console.log(
  [[1, 2, 3], [4, 5], [6]].my.flatten()
)

console.log(
  [1, 2, 3].my.flatMap(x => [x, x])
)