I want to delete a node from a graph in JanusGraph but I don't know what will happen to its edges? Will they be orphaned? or will be deleted?
The low level details depend on which backend you using. I think most end-up deleting the edges all together.
For the Cassandra backend edges will be tombstoned and will later get cleaned up (more info here). This means that at first the edges will be hidden for a while and later the data records will be removed.
Either way you won't be able to see those edges.
I am not 100% sure for all the other backends.