/**
 *
 * @param arr is a nested or one dimensional array
 * @returns an one dimensional array or undefined
 */
const flatten = (arr) => {
  if(!Array.isArray(arr)) {
    console.log(`flatten parameter should be an array but "${arr}" was passed`)
    return undefined
  }
  const flattenReducer = (acc, item) => {
    const flattenedItem = Array.isArray(item) ? flatten(item) : item
    return acc.concat(flattenedItem)
  }
  return arr.reduce(flattenReducer, [])
}

// const deeplyNestedArr = [[1,2,3,[9],[1,2,3,[9]]],4]
// console.log(flatten(deeplyNestedArr))
// console.log(flatten(1))
// console.log(flatten({}))