sumUpDiagonals

const arr = [
  [1,2,3,4],
  [4,5,6,5],
  [7,8,9,6],
  [1,4,6,2]
];

const sumUpDiagonals = arr => {
  
  let sumOfFirstDiagonal = 0;
  let sumOfSecondDiagonal = 0;
  let sumOfBothDiagonals = 0;
  const lengthOfArray = arr.length;
  
  for (let i = 0; i < lengthOfArray; i++) {
    sumOfFirstDiagonal += arr[i][i];
  }
  
  for (let i = lengthOfArray; i > 0; i--) {
    sumOfSecondDiagonal += arr[i - 1][lengthOfArray - i];
  }
  
  sumOfBothDiagonals = sumOfFirstDiagonal + sumOfSecondDiagonal;
  
  return sumOfBothDiagonals;
  
}

console.log(sumUpDiagonals(arr));