Javascript Reduce

const list = [1, 2, 3, 4, 5]
const initValue = 10
const totalSum = list.reduce(
  (accumulator, currentValue, currentIndex, array) => {
    return accumulator + currentValue
  },
  initValue,
)

25

const complicatedList = [[0, 1], [2, 3], [4], [5, 6]]
complicatedList.reduce(
  (accumulator, currentValue) => accumulator.concat(currentValue),
  [],
)

[0, 1, 2, 3, 4, 5, 6]

const moreComplicatedList = [[0, 1], [[[2, 3]]], [[4, 5]], 6]

const flatten = function (arr, result = []) {
  for (let i = 0, length = arr.length; i < length; i++) {
    const value = arr[i]
    if (Array.isArray(value)) {
      flatten(value, result)
    } else {
      result.push(value)
    }
  }
  return result
}

flatten(moreComplicatedList)

[0, 1, 2, 3, 4, 5, 6]