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 }