fix(core): add support to union types for `DeepValue`

[irwinarruda]

Solves #691

Explanation

@Balastrong provided two issues in the example. Only one of those is a bug. Consider the following code:

type Text = { answer: string; textAnswer: string }
type Number = { answer: number }
type FormType = { questions: Array<Text | Number> }

• When calling DeepValue<FormType, 'questions[0].textAnswer'> the result is unknown. This is a bug!
• When calling DeepValue<FormType, 'questions[0].answer'> the result is number | string. This is not a bug because Typescript will never know the current state of the if checks.

Solution

As I explained on the issue page, the problem was solved by swapping the default typescript get function with a custom one.

Problems found along the way

Recently, the #713 and #717 were merged. From what I gathered, if there is a nullable union, all the properties from the object part will also be nullable. So for example:

type Obj = { a: { b: string } | null }
// DeepValue<Obj, 'a.b'> -> string | null

In order to create a standardized behavior, I decided to ask you some questions.

• Why is it only null values? What about optional and undefined?
• Shouldn't it work with deep nested values?

DeepValue<{ a: { b: { c: string } } | null }, 'a.b.c'>
// should be -> string | null but is only string

• Should the object union work the same way? So for the first example I gave:

DeepValue<FormType, 'questions[0].textAnswer'>
// should be -> string | undefined instead of just string (which is the way I implemented it)

Please let me know if you would like to chat more about it. I recently joined the Tanstack Discord.