"Simple First Draft": addWeirdStuff.js

const addWeirdStuff = (arrayOne, arrayTwo) => {
  let sumOfEvensInArrayTwo = 0
  let sumOfOddsInArrayTwo = 0
  let addOneToArrayOne = 0 // if an element of arrayTwo is over 20
  for (const itemTwo of arrayTwo) {
    if (itemTwo % 2 === 0) {
      sumOfEvensInArrayTwo += itemTwo
    }
    if (itemTwo % 2 === 1) {
      sumOfOddsInArrayTwo += itemTwo
    }
    if (itemTwo > 20) {
      addOneToArrayOne = 1
    }
  }
  const outputArray = []
  for (const itemOne of arrayOne) {
    if (itemOne < 10) {
      outputArray.push(itemOne + sumOfOddsInArrayTwo + addOneToArrayOne)
    }
    if (itemOne >= 10) {
      outputArray.push(itemOne + sumOfEvensInArrayTwo + addOneToArrayOne)
    }
  }
  return outputArray
}

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