code from episode

vanilla-hofs-with-reduce.js

const isMoreThan5 = number => number > 5
const numbers = [ 2, 4, 8, 9 ]
const result = filter(isMoreThan5, numbers) 
const addThree = number => number + 3
const result = map(addThree, numbers)
result

function map(transform, array) {
  const initialArray = []
  const mapReducer = (mappedArray, currentItem) => 
    mappedArray.concat(transform(currentItem))
  return reduce(mapReducer, initialArray, array)
}

function filter(predicate, array) {
  const initialArray = []
  const filterReducer = (filteredArray, currentItem) => {
    const shouldBeIncluded = predicate(currentItem)
    if(shouldBeIncluded) {
      return filteredArray.concat(currentItem)
    }
    return filteredArray
  }
  return reduce(filterReducer, initialArray, array)
}

function reduce(reducer, initialAccumulatorValue, array) {
  let accumulatorValue = initialAccumulatorValue
  for (let i = 0; i < array.length; i++) {
    const current = array[i]
    current
    accumulatorValue
    accumulatorValue =
      reducer(accumulatorValue, current)  
    accumulatorValue
  }
  return accumulatorValue 
}

Slight rewrite to make it work with sparse (ie. arrays with holes eg. [2, , 4, 8, 9]) of course I don't recommend using sparse arrays but javascript arrays can have holes. I also added in the additional parameters to the callback functions the index and the array itself that are in the native array methods.

otherwise for numbers = [2, , 4, 8, 9] you get mapped result of [5, nan, 7, 11, 12] when you should get [5, , 7, 11, 12] with the hole preserve

for more info see the blog post An adventure in sparse arrays by Remy Sharp.

const isMoreThan5 = number => number > 5
const numbers = [2, 4, 8, 9]
const result = filter(isMoreThan5, numbers)
const addThree = number => number + 3
const result2 = map(addThree, numbers)

function map(transform, array) {
  const initialArray = Array(array.length)
  const mapReducer = (mappedArray, currentItem, currentIndex) => {
    mappedArray[currentIndex] = transform(currentItem, currentIndex, array)
    return mappedArray
  }
  return reduce(mapReducer, initialArray, array)
}

function filter(predicate, array) {
  const initialArray = []
  const filterReducer = (filteredArray, currentItem, currentIndex) => {
    const shouldBeIncluded = predicate(currentItem, currentIndex, array)
    if (shouldBeIncluded) {
      return filteredArray.concat(currentItem)
    }
    return filteredArray
  }
  return reduce(filterReducer, initialArray, array)
}

function reduce(reducer, initialAccumulatorValue, array) {
  let accumulatorValue = initialAccumulatorValue
  for (let i = 0; i < array.length; i++) {
    if (array.hasOwnProperty(i)) {
      const current = array[i]
      accumulatorValue = reducer(accumulatorValue, current, i, array)
    }
  }
  return accumulatorValue
}