UnknownMaybe: utility type that treats an object as "unknown but might be of type T"

UnknownMaybe.ts

// utility type that treats an object as "unknown but might be of type T"
// allows you to access each property that T has but assumes the value is `unknown | undefined`
// this lets you test the values of each property to determine if it is in fact a T
// autocomplete and refactoring the property names work
// you get a compiler error if you try to access a property not in T
type UnknownMaybe<T> = {
  [P in keyof T]?: T[P] extends object ? UnknownMaybe<T[P]> : unknown
}

interface Message {
  content: string
  meta: {
    timestamp: Date
  }
}

function handle(msg: UnknownMaybe<Message>): void {
  // if msg was just "unknown" you could not access any properties
  if (typeof msg.content === 'string' && typeof msg.meta?.timestamp === 'string') {
    // this is a valid Message
  }
}