Refer to Function type retaining generics

Question

In Typescript, is there a way to refer to a function type with generics, without having to instantiate the generics? (Passing them to be instantiated when the function is actually called).

For example, I have the following type:

type FetchPageData<T> = (client : APIClient<T>) => T;

where APIClient is an abstract class like this:

export abstract class APIClient<T> { abstract getData(demoDataIsSelected : boolean, queryParameters ?: ApiQueryParameters) : T; }

I want to create a function of this type, but if I try:

fetchPageData : FetchPageData = client => { return client.getData(x, y); }

Typescript requires that I provide the generic type for T. What I want is similar to what we do when refering to a function definition, instead of calling it. Similarly I would expect this to refer to the type definition without requiring the T, because the T concerns only the calling of the function.

If not like this, what is the syntax for me to get a T and pass it to FetchPageData on the instantiation? (It has to be with const syntax, instead of function, because I'm actually asigning with useCallback on react)

Thiago Pereira Maia · Accepted Answer · 2021-09-15 12:18:44Z

I figured it out right after posting this:

By putting the <T> before the equal sign, I was using T as a generic for the creation of the function, instead of defining the FetchPageData as a function type that takes a generic argument. The correct syntax would be:

type FetchPageData = <T>(client : APIClient<T>) => T;

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