For In To Iterate Sparse Array

I know that using for in loop to iterate over array elements is a bad
idea but I have a situation where it is tempting me to use it. I have
a two dimensional sparse array say "arr" which will have length to be
some 50 but will have some elements like arr[1], arr[10], arr[23] at
random locations. Each of these elements is again an array (since it
is two dimensional) say arr[1][30], arr[1][24] & so on for arr[10 &
23] also. Currently I've coded it like this:

for (var i = 0; i < arr.length; i++) {
for (var j = 0; j < arr[i].length; j++) {
if (arr[i] && arr[i][j]) {
// do stuff with arr[i][j]
}
}
}

but i am tempted to do following

for (var i in arr) {
if (!arr.hasOwnProperty(i)) {
continue;
}
for (var j in arr[i]) {
if (arr[i].hasOwnProperty(j)) {
// do stuff with arr[i][j]
}
}
}

thinking it to be faster. What are your views?