Looking at the Surprise rules, I wonder, does a character even have a reaction to take before the start of their first turn? I don't have XGtE in front of me.

— Heath Belden (@HeathBelden) November 14, 2017

A number of effects in the game deprive you of the ability to take reactions: being surprised or incapacitated, for example. If you're not suffering such an effect, you can take a reaction before your first turn in a combat. #DnD https://t.co/tfkNchP4xx

— Jeremy Crawford (@JeremyECrawford) November 14, 2017