javascript - How can I call a global function in React?

Yesterday I had an interview for React developer position. Interviewer asked me about global functions and how to create and call global functions with React ?

I thought he is talking about Redux and state containers ...

But he said, no ... it's not about state containers and it's about react services.

Since after interview I searched about react services, but I didn't find any thing.

What do you think about this feature and global functions ? I should import global functions above all components or here is different method to handle this ?

Thanks