I'd rather have a default statement then having it set before the switch

pass by reference?
I suppose it would make sense here as we'd potentially have a lot of endpoints.

I see you made $this->push_batch() receive the parameter by reference, which is fine, and actually something I missed ;)
What I actually meant before was the foreach

We generally don't do this in Lunr. Type hinting provides contracts, and Lunr mostly provides contract free functionality.

Ship It!