Support `exactOptionalPropertyTypes` for `partial` and `optional` helpers

Hi!

Zod is adding the undefined type to all keys that are optional when using the different .partial() modifiers.

How about differentiating them by being present and undefined or just not present at all like typescript does with exactOptionalPropertyTypes: true.

Would it be possible to implement in zod? I guess it could be the default since if you don’t care you don’t care. Also just using the typescript Partial type will actually honor exactOptionalPropertyTypes by default (on the type level that is).

[akomm]

@zetaraku I think the problem is that while everything is fine type-wise when doing so, the code that runs at runtime need to know whether an explizit undefined value is acceptable. When exactOptional... is enabled, the expected runtime behavior for key?: number would be to fail parsing when key exists, but is undefined. But the runtime does not know under which TS settings the code was compiled. This is why it appears to me there is no way to signal this intention to the runtime besides via a separate refinement function that has this behavior AND typing.

[jameshfisher]

Moving my complaint in https://github.com/colinhacks/zod/issues/1540 here, because it sounds like the same issue.

My specific problem is that I’m trying to parse values into this type:

type FormattedText = { text: string; bold?: true; };

The bold flag, if present, is always the value true. Example test cases:

// Valid values include
{ text: "foo" }
{ text: "foo", bold: true }

// Invalid values include
{ text: "foo", bold: false }
{ text: "foo", bold: undefined }

This schema is designed to work with Slate, and the values are in a database. The design is therefore non-negotiable.

I can’t figure out how to configure zod to make the bold property optional, without also adding undefined to the type of the property. The .optional() method seems to do conflate the two: it makes the property optional, but also allows the value undefined if present. For example, zod.parse allows the value { text: "foo", bold: undefined }.

I realize I could do this with .refine(), but then the constraint is not represented in the type system. I want the type of .parse() to be { text: string; bold?: true; }.