TypeScript: deep partial?

Question

You can simply create a new type, say, DeepPartial, which basically references itself:

type DeepPartial<T> = {
    [P in keyof T]?: DeepPartial<T[P]>;
};

Then, you can use it as such:

const foobar: DeepPartial<Foobar> = {
  foo: 1,
  bar: { baz: true }
};

See proof-of-concept example on TypeScript Playground.

If you're looking for a quick and easy solution, check out the type-fest package, which has many useful prebuilt TypeScript types including the PartialDeep type.

For a more technical and customizable solution, see this answer.

I inspired myself on the answers on this question to create my own version of PartialDeep.

I stumbled upon some issues with built-in objects along the way; for my use case, I wouldn't expect a Date object to be missing some of its methods. It's either there, or it isn't.

Here's my version:

// Primitive types (+ Date) are themselves. Or maybe undefined.
type PartialDeep<T> = T extends string | number | bigint | boolean | null | undefined | symbol | Date
  ? T | undefined
  // Arrays, Sets and Maps and their readonly counterparts have their items made
  // deeply partial, but their own instances are left untouched
  : T extends Array<infer ArrayType>
  ? Array<PartialDeep<ArrayType>>
  : T extends ReadonlyArray<infer ArrayType>
  ? ReadonlyArray<ArrayType>
  : T extends Set<infer SetType>
  ? Set<PartialDeep<SetType>>
  : T extends ReadonlySet<infer SetType>
  ? ReadonlySet<SetType>
  : T extends Map<infer KeyType, infer ValueType>
  ? Map<PartialDeep<KeyType>, PartialDeep<ValueType>>
  : T extends ReadonlyMap<infer KeyType, infer ValueType>
  ? ReadonlyMap<PartialDeep<KeyType>, PartialDeep<ValueType>>
  // ...and finally, all other objects.
  : {
      [K in keyof T]?: PartialDeep<T[K]>;
    };