uchat | 有聊：基于兴趣分群的匿名聊天APP（react-native redux react-router） | Frontend Framework library

The Firebase client always fires with the complete snapshot of the data at a location. So if you get this output:

DataSnapshot { key = kk44p2XW97OIxAudLwekOZwZM7B2, value = {lastSeen=-1} }

There is some code that writes to kk44p2XW97OIxAudLwekOZwZM7B2 with just a lastSeen value. You might want to check your code for calls to setValue(), as that would explain why the previous data under kk44p2XW97OIxAudLwekOZwZM7B2 disappeared in that second snapshot.