Really depends on the whole structure. If it is mainly FP, dependency rejection usually is actually the easiest to test with

I dunno how you can combine dependency injection (as I imagined it in the original comment) with FP in the first place.

Without classes what would you be injecting? Other functions? That’s still something that happens all the time in FP.

I was thinking sideeffectful functions. Such as DB accesses.

> dependency rejection

That’s secretly the most powerful design strategy.

It objectively isn't, as there are certain things you can't do with it.

Altough, for usability it is IMO definitely the best.