interface User {
  username: string
  email: string
}
const user1: User = { username: 'x', email: 'y', foo: 'z' } //=> Currently errors when `foo` is unknown.
const user2: Exact<User> = { username: 'x', email: 'y', foo: 'z' } //=> Still errors with `foo` unknown.
// Primary use-case is when you're creating a new type from expressions and you'd like the
// language to support you in ensuring no new properties are accidentally being added.
// Especially useful when the assigned together types may come from other parts of the application 
// and the result may be stored somewhere where extra fields are not useful.
const user3: User = Object.assign({ username: 'x' }, { email: 'y', foo: 'z' }) //=> Does not currently error.
const user4: Exact<User> = Object.assign({ username: 'x' }, { email: 'y', foo: 'z' }) //=> Will error as `foo` is unknown.

type Foo = Exact<X> | Exact<Y>
type Bar = Exact<{ username: string }>
function insertIntoDb (user: Exact<User>) {}