<blockquote>
callbacks.js
intro.js & outro.js
</blockquote>
It's not the same, here is an assignment of FE result and parentheses aren't necessary, in all of those cases parentheses should be present and there is no assignment. 

<blockquote>
Yes, precisely.
</blockquote>
Got it, yeah, i suppose it might not be visible from first look, will revert it shortly.