select and deselect multiple items from array

From your code, I was quite confused regarding the difference between this.state.selGame.friends and this.state.friendsChat. Maybe I missed something in your explication. However, I felt that your code seemed a bit too overcomplicated for something relatively simple. Here’s my take on that task:

class Game {
  state = {
    friendsChat: [] as string[],
  };

  onFriendToggle(key: string) {
    const gameRoomMembers = this.state.friendsChat;

    if (gameRoomMembers.includes(key)) {
      this.state.friendsChat = gameRoomMembers.filter(
        (member) => member !== key
      );
    } else {
      this.state.friendsChat = [...gameRoomMembers, key];
    }
  }
}

I used typescript because it makes things easier to see, but your JS code should probably give you a nice type inference as well. I went for readability over performance, but you can easily optimize the script above once you understand the process.

You should be able to go from what I sent you and tweak it to be according to what you need