owners: [
  {
    userID: "58c4d7ac",
    username: "John.Doe",
    firstName: "John",
    lastName: "Doe",
    email: "John.Doe@acme.com"
  },
  {
    userID: "68c4d7ac",
    username: "User2.Name2",
    firstName: "User2",
    lastName: "Name2",
    email: "dh@acme.com"
  }
]

users: [
  {
    userID: "58c4d7ac",
    username: "John.Doe",
    firstName: "John",
    lastName: "Doe",
    email: "John.Doe@acme.com"
  },
  {
    userID: "68c4d7ac",
    username: "User2.Name2",
    firstName: "User2",
    lastName: "Name2",
    email: "dh@acme.com"
  },
  {
    userID: "88c4d7ac",
    username: "User3.Name3",
    firstName: "User3",
    lastName: "Name3",
    email: "dh@acme.com"
  }
]

const usersItems = users.map(user => {
    // Check whether the user is already an owner
    if (owners.findIndex(owner => owner.userID === user.userID) === -1) {
        return  owner
    } else {
        return null;
    }
});

console.log(usersItems);

// Filter out all items which are null
const newUsersItems = usersItems.filter(user => {
    if (user) return user;
});

console.log(usersItems);

newUsers: [
  {
    userID: "88c4d7ac",
    username: "User3.Name3",
    firstName: "User3",
    lastName: "Name3",
    email: "dh@acme.com"
  }
]