Oh nice, I did this too in Javascript! I was afraid it wouldn't work because the pattern "seemed" to be that the x/y coordinates added up to be 1 less than N.

var backSlash = 0;
var forwardSlash = 0;
for (var i = 0; i < n; i++) {
    for (var j = 0; j < n; j++) {
        if (i == j) {
            backSlash += a[i][j];
        }
        if (i + j == n-1) {
            forwardSlash += a[i][j];
        }
    }
}

console.log(Math.abs(backSlash - forwardSlash));