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

function sumUpDiagonals(matrix) {
    var diagonals = [];
    for (var i=0; i < matrix.length; i++){
        diagonals.push(matrix[i][i]);
        diagonals.push(matrix[i][matrix.length-i-1]);
    }
    return diagonals.reduce(function (x, y){
        return x+y;
    });
}

console.log(sumUpDiagonals(matrixExample));