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Inference is not working properly #63348
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Activity
seems to be that the issue is when the first generic type is specified and we expect that the second argument will be inferred from the function parameters
This expectation is not correct. If a type argument is given, the rest of the parameter go to their defaults, not inference. See https://www.typescriptlang.org/docs/handbook/2/generics.html#generic-parameter-defaults
Ryan Cavanaugh (@RyanCavanaugh) thanks for your time, is there a way to make this works ? or do i need to re-think my whole solution ?
or can be posible to make a proposal ? in that way we can avoid be duplicating the value of the generic and in the argument of the function twice like u can see in my example instead go automatically to the default value
Suggestion is tracked at #26242 btw.
I don't have a good sense of what your intended API design is here. Like where is
stylecoming from?Ryan Cavanaugh (@RyanCavanaugh) im trying to make a global store, where the first argument is the state type
{ schema: { subValue: number }; style: string }and the second generic is the path based on dots, for exampleschema.subValueso the function will give me that value.in the function parameters TS should predict all of the possible paths ['schema', 'schema.subValue', 'style',....], so i have to specify it
of course the function is dynamic based on the state that needs to manage, thats why was expecting to specify the first generic and leave the second to be infered based on the first parameter in the function
pathif is required i can give you the full code
Note,
due this limitation i have tu put twice the generic and the function parameter value, for example
// type function function useStoreByPath<TState extends object, P extends PathOf<TState>>( path: P, equalityFn?: (a: PathValue<TState, P>, b: PathValue<TState, P>) => boolean ): [ PathValue<TState, P>, (value: PathValue<TState, P> | ((prev: PathValue<TState, P>) => PathValue<TState, P>)) => void ]; // example const [subValue, setSubValue] = useStoreByPath<{ schema: { subValue: number }; style: string }, 'schema.subValue'>('schema.subValue');due that the second generic is not infered, if i dont put it, TS complains
Expected 2 type arguments, but got 1.The standard workaround is to use a curried function:
function useStoreByPath<TState extends object>() { return <P extends PathOf<TState>>( path: P, equalityFn?: (a: PathValue<TState, P>, b: PathValue<TState, P>) => boolean ): [PathValue<TState, P>, (value: PathValue<TState, P> | ((prev: PathValue<TState, P>) => PathValue<TState, P>)) => void] => { // implementation }; } // Usage: specify TState manually, P is inferred const [subValue, setSubValue] = useStoreByPath<{ schema: { subValue: number }; style: string }>()('schema.subValue');
for a library perspective wont looks good :/, can be possible to request a change request or suggestion ? or could break how typescript manage the generics ?
just in case, thanks for the help if is not possible will try to find another way
can be possible to request a change request or suggestion ?
He already linked the relevant existing issue: #26242
Reacted by Carlos RodriguezThis issue has been marked as "Working as Intended" and has seen no recent activity. It has been automatically closed for house-keeping purposes.
Ryan Cavanaugh (@RyanCavanaugh) hi, a question if is working as expected, instead reopen this one, should i wait for #26242 ?
Regards
- locked as resolved and limited conversation to collaborators
on Jun 5, 2026
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inference
performance
⏯ Playground Link
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🙁 Actual behavior
im testing some hard inference, and seems to be that typescript is not able to infer properly the generic type from the function parameter, so i have to specify it to make it able to works
for example
🙂 Expected behavior
if i specify the first argument of my function, this should automatically infer it
for example
i have attached a link where can be reproduced
Additional information about the issue
seems to be that the issue is when the first generic type is specified and we expect that the second argument will be inferred from the function parameters