Is it possible for TypeScript to infer the argument type of a generic function with a return value?

Question

Is it possible for TypeScript to infer the argument type of a generic function with a return value?

I am currently working on creating a versatile function that can take another function and deduce the return type automatically (similar to how map functions). Here is the code I have so far:

type game_selector<T> = <T>( state : GT.game_state ) => T

export function useGameState<T>( gs : game_selector<T> ): T {

When calling this function, it looks like this:

const users = useGameState( gs => gs.users )

Based on the error message received, it seems that the type of gs is correctly inferred since it recognizes gs.users as Users, however, the following error occurs:

TS2322: Type 'Users' is not assignable to type 'T'

What would be the correct way to specify the typing for this function?

typescript typescript-generics

Answer 1

Answer №1

You've included an unnecessary additional generic type in this code snippet:

type game_selector<T> = <T>( state : GT.game_state ) => T;

The returned T should actually be the same as the one specified in game_selector<T>, not an extra <T>(...).

If you test this in the Typescript playground, you'll notice a slightly greyed-out T within game_selector<T>. Hovering over it will display a warning message:

'T' is declared but its value is never read.(6133)

To correct this issue, simply make the following adjustment to your code:

type game_selector<T> = ( state : GT.game_state ) => T;

Answer 2

You've included an unnecessary additional generic type in this code snippet:

type game_selector<T> = <T>( state : GT.game_state ) => T;

The returned T should actually be the same as the one specified in game_selector<T>, not an extra <T>(...).

If you test this in the Typescript playground, you'll notice a slightly greyed-out T within game_selector<T>. Hovering over it will display a warning message:

'T' is declared but its value is never read.(6133)

To correct this issue, simply make the following adjustment to your code:

type game_selector<T> = ( state : GT.game_state ) => T;

Is it possible for TypeScript to infer the argument type of a generic function with a return value?

Answer №1

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