Sequence diagram – modelling HTTP requests

For my school project, I need to create a sequence diagram. In that sequence diagram I have to represent an API call using simple JavaScript code (fetch() + then() for handling the response).

I’m confused, should I represent this as sync or async communication in a sequence diagram?

In one way, since the web browser is not blocked while waiting for a response I guess it should be async. But in another way, it waits for the response so it can be processed so it’s blocked from executing which means is should be sync?

Thanks for the answers.

Picture that represents sync and async communication

I tried google-ing this problem but didn’t find any answers.