return increase or decrease arrays number

let arr = [1,2,3,4]
function increase(arr){
  newArr = []
  for(let i=0; i<arr.length; i++){
    newArr.push(arr[i]+1)
  }
  return newArr
}

function decrease(arr){
  newArr = []
  for(let i=0; i<arr.length; i++){
    newArr.push(arr[i]-1)
  }
  return newArr
}