Cannot infer types on arrays of union of objects
Balastrong opened this issue · comments
Describe the bug
The library cannot infer the right type of fields in a (probably weird) edge case, when an array is an union type of objects.
Take this as example:
type Text = {
type: 'text';
answer: string;
textAnswer: string;
};
type Number = {
type: 'number';
answer: number;
};
type FormType = {
questions: Array<Text | Number>
}
When accessing to questions[0].answer
(field in both types) the type is string | number
.
When accessing to questions[0].textAnswer
(field on one type only) the type is unkown
Your minimal, reproducible example
https://stackblitz.com/edit/tanstack-form-ydorgn?file=src%2Findex.tsx%3AL53-L53
Steps to reproduce
- Have a structure with an array of union of objects
- Use field.state in jsx
Expected behavior
Not sure if it's a bug or a missing feature, but I'd like a way to know which is the correct type of the array entry and have all the fields typed accordingly.
The form is handling the values right, typescript isn't.
See the MRE for more comments.
How often does this bug happen?
Every time
Screenshots or Videos
No response
Platform
macOS
TanStack Form adapter
react-form
TanStack Form version
v0.19.4
TypeScript version
No response
Additional context
No response
I found this problem interesting so I investigated a little. From what I've gathered, the issue revolves around the DeepValue
type and happens because of a default typescript behavior with union types:
type A = { a: string; };
type B = { b: number; };
type C = boolean;
// Getting a value from a union causes some problems and the DeepValue uses this approach
type result1 = (A | B)["a"]; // any
type result2 = (B | C)["b"]; // any
My idea to solve it boils down to creating a new Get
type that behaves differently from the default typescript one:
type Get<T, K extends string /* or keyof T*/> = T extends {
[Key in K]: infer V;
} ? V : never;
type result1 = Get<A | B, "a">; // string
type result2 = Get<B | C, "b">; // number
Though I created it, I don't fully know how the Get
type works because it sometimes behaves strangely, so this solution feels a bit hacky. Look at what I found changing the else
clause. It might be a typescript
bug.
type Get<...> = T extends { ... } ? V : unknown;
type result1 = Get<A | B, "a">; // unknown
type result2 = Get<B | C, "b">; // unknown
type Get<...> = T extends { ... } ? V : undefined;
type result1 = Get<A | B, "a">; // string | undefined
type result2 = Get<B | C, "b">; // number | undefined
type Get<...> = T extends { ... } ? V : never;
type result1 = Get<A | B, "a" | "b">; // never
@crutchcorn, if you and the team think it's a good idea to continue with this issue, I would like to create a PR to solve it. Please let me know if I should.
@irwinarruda please do open a PR! :) We have extensive type tests, so we'll need to make sure they all pass, but any help here would be greatly appreciated!