var x = 5,
y = 5,
arr = [],
add;
for (i = 0; i < x; i++) {
arr[i] = new Array();
for (j = 0; j < y; j++) {
    arr[i][j] = Math.round(Math.random() *(10 - 1) + 1);
}
if (add === undefined) {
    add = arr[i][i]
    console.log('1st add = ' + add);
} else {
    add *= arr[i][i]
}
}
console.log(add);
console.log(arr);