{ Code: Impossible }

I'm poking around in the JSOQ source this week and came across this gem.

var array = [
    { id: 1, num: 2 },
    { id: 2, num: 3}
];

// find the item with num == 2
for(var item in array) {
    if( item.num == 2 ) {
        alert(item.id);
    }
}

The code above isn't calling alert(item.id). Why is this?