(i) or ([i])??

I am trying a combo. I realized I can use

combo.setSelectedItems(i);
combo.setCurrentItem(i);

or,

combo.setSelectedItems([i]);
combo.setCurrentItem([i]);

No errors on both usage. If I use combo.setCurrentItem([i]), I always get 0 when using combo.getCurrentItem(). That is only difference I know. Could you explain all differences for both usages?

Thanks,

setSelectedItems([i]); // expects an array of item indices
setCurrentItem(i); // expects single index