var a = ['bar'];
_.observe(a, 'update', function(new_item, old_item, item_index) {
console.log(new_item + ' was changed from ' + old_item + ' at ' + item_index);
});
// should log "foo was changed from bar at 0"
a[0] = 'foo';
// Prevents 'foo' from being picked up
_.observe(a, 'delete', function() { });