Make a TypeScript interface completely optional

optional.ts

// Optional declares a type of keyed type T where all keys are optional.
// This allows 
type Optional<T> = { [P in keyof T]? :T[P] }
  
interface Foo {
  x :number
  y :number
}

type OptionalFoo = Optional<Foo>

// OptionalFoo == interface Foo {
//   x? :number
//   y? :number
// }

function fooify(extraFoo :OptionalFoo) :Foo {
  const baseFoo :Foo = getBaseFoo()
  return Object.assign({}, baseFoo, extraFoo)
}

fooify({x:3}) // returns a valid Foo
fooify({z:4}) // error: no "z" property in OptionalFoo
fooify({y:'5'}) // error: bad type "string" for property y; expected "number"

Built in Partial:
https://www.typescriptlang.org/docs/handbook/utility-types.html#partialtype

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