Let's say I have a list of friend objects:

myFriends: { name: string, friends: friend[] }[]

If I wanted to get a list of friends of friends, I could do something like

myFriends.map(prop('friends')).flatten()

Or

myFriends.flatMap(prop('friends'))

My data is a list of lists but I really want a list of friends so I flatten.

Maybe the Friend object is poorly created/designed but often times you'll just have to deal with whatever the API gives you.