Create a function that receives a (square) matrix and calculates the sum of both diagonals (main and secondary)

Diagonals sum

// [
//  [ 1, 2, 3 ],
//  [ 4, 5, 6 ],
//  [ 7, 8, 9 ]
// ] 

function sum(matrix) {
  return matrix.reduce((sum, value, index) => sum + value[index] + value[value.length - index -1], 0);
}