How Array.prototype.reduce works

reduce.js

const poop = ['one', 'two', 'three', 'four', 'five']

// I always use `acc` and `item` by default
// acc is the thing being accumulated
// item is each item in `poop`

// basic idea: this will do nothing,
// add some console.logs in here and study the behaviour
// look at the example below that uses `i` and add it here with console.log
const ultraBasic = poop.reduce((acc, item) => {
  acc.push(item)
  return acc
}, [])
console.log('ultraBasic', ultraBasic)

// now study this one
const accumulateObj = poop.reduce((acc, item) => {
  acc[item] = item
  return acc
}, {})
console.log('accumulateObj', accumulateObj)


// now study this one
const accumulateNum = poop.reduce((acc, item, i) => {
  acc += i
  return acc
}, 0)
console.log('accumulateNum', accumulateNum)

// now study this next-level swiss-army knife
const accumulateSavageUnit = poop.reduce((acc, item) => {
  if (item.length < 4) acc.short.push(item)
  if (item.length >= 4) acc.long.push(item)
  return acc
}, { short: [], long: [] })
console.log('accumulateSavageUnit', accumulateSavageUnit)

// key mentions:
// 1) notice how the second param of reduce is the initial state
// 2) notice how it looks like map except the accumulator comes in
// every time along with the item
// 3) notice how the accumulator is returned every iteration
// 4) notice how it ends with returning the completed accumulation-product
// 5) notice how you can imagine a million ways to use this type of process