"Simple First Draft": addWeirdStuffMap.js

const addWeirdStuffMap = (arrayOne, arrayTwo) => {
  const [sumOfEvensInArrayTwo, sumOfOddsInArrayTwo] = arrayTwo.reduce(
    ([sumOfEvens, sumOfOdds], itemTwo) => {
      return itemTwo % 2 === 0
        ? [sumOfEvens + itemTwo, sumOfOdds]
        : [sumOfEvens, sumOfOdds + itemTwo]
    },
    [0, 0] // Start with an initial value of 0 for both sums
  )
  // Add one to each itemOne if arrayTwo contains an element greater than 20
  const addOneToArrayOne = arrayTwo.some((itemTwo) => itemTwo > 20) ? 1 : 0
  return arrayOne.map((itemOne) =>
    // Add sum of odds if itemOne is less than 10; add sum of evens if >= 10
    itemOne < 10
      ? itemOne + sumOfOddsInArrayTwo + addOneToArrayOne
      : itemOne + sumOfEvensInArrayTwo + addOneToArrayOne
  )
}

console.log("------------")
console.log(
  `[${addWeirdStuffMap(
    [1, 3, 5, 17, 15],
    [1, 2, 3, 4, 5]
  )}] should equal [10,12,14,23,21]`
)
console.log(
  `[${addWeirdStuffMap(
    [1, 3, 5, 17, 15, 1],
    [1, 2, 3, 4, 5, 22]
  )}] should equal [11,13,15,46,44,11]`
)