Retrieve property of [object Map] with arrow function - JS

Elo,

my issue is the following...

I have a map in js, which was created through a fetch(). This map however includes an arrow function, of which I can't grasp how to access one of its properties/values

Map(1) { '<id>' => User { id: '<id>', bot: false, system: false, flags: UserFlagsBitField { bitfield: 0 }, username: 'someUserName', globalName: 'someGlobalUsername', discriminator: '0', avatar: '<numbers>', banner: null, accentColor: null, avatarDecoration: null, avatarDecorationData: null, collectibles: null, primaryGuild: null } }

Additionally, when this map is returned through fetch, both <id>'s are always the same if that is of any help?
This is the discord.js result, after fetch().

Now in my case, I need to somehow access that map's username (or rather its value).
I've tried more fetching, something like "let someVar = thisVar.username;" or whatever
js' maps.get() as well, in various ways.
Idek anymore, send help. No results when googling either.

If any additional code is needed (although I doubt), tell me.