interface LoginInterface {
  username: string;
  password: string;
}

const MyObject = {
  credentials: null as LoginInterface,
  otherInfo: "I'm something not relevant to this question"
}

const MyObject: {
  credentials: LoginInterface,
  otherInfo: string
} = {
  credentials: null,
  otherInfo: "I'm something not relevant to this question"
};

const MyObject: { credentials: ?LoginInterface } = {
  credentials: null,
  otherInfo: "I'm something not relevant to this question"
}

type MyType = { credentials: ?LoginInterface, otherInfo?: string }