sumOfBothDiagonals.js

function sumDiagonal(a){
  const n = Math.sqrt(a.length);
  const op = { d1: 0, d2: 0 };
  for(let i = 0; i < n; i++){
    op.d1 += a[i + n * i ];
    op.d2 += a[n - i - 1 + n * i];
  }
  return op;
}

const input = [
  1,  2,  3,  4,
  5,  6,  7,  8,
  9,  10, 11, 12,
  13, 14, 15, 16,
]

console.log(sumDiagonal(input));