Сумма основной и побочной диагонали матрицы

exxample1.txt

var matrixExample = [
   [ 1, 2, 3, 4 ],
   [ 4, 5, 6, 5 ],
   [ 7, 8, 9, 7 ],
   [ 7, 8, 9, 7 ]
 ];

function sumUpDiagonals(matrix) {
  return matrix.reduce(function(prev,current,index,arr) {
    return prev + current[index] + current[arr.length-1-index]
  }, 0);
}

console.log(sumUpDiagonals(matrixExample))