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A lib I'm using exports a function with generic types, but not the type itself, something like this,

// some-lib/index.d.ts
export function libFunction<T>(): InternalLibType<T>;

I'd like to declare a variable of type

const foo: InternalLibType<"something">;

I'm trying to construct the desired type for foo using ReturnType<typeof libFunction> as a starting point, but don't really know how to continue from here.

How can I create a concrete type from the generic type? To be clear, the lib does not export InternalLibType.

The library in question is use-debounce. While it does technically export the type from one of its files, the exported type is not in a file declared in the exports field, thus causing errors with some tools that respect the exports field.

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This line export function libFunction<T>(): InternalLibType<T>; means that libFunction expects explicit generic during the call (which is, AFAIK, unsafe in most cases).

This is not a constructor type which expects a generic argument. It means that you are not allowed to use ReturnType<typeof libFunction<'something'>>.

If you want to infer return type of this function, you have two options:

First, call this function:

const result = libFunction<'something'>();

type Foo = typeof result;

Second, just use InternalLibType<"something">.

SInce TS has a structural type system, type Foo is equal to InternalLibType<"something">

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10 Comments

The second options would be ideal, unfortunately, InternalLibType is not exported
Could you please provide me with a link to third party library? SOme times it is possible to obtain this type from the other one
Added more details in the original post to provide more context than I can with a comment.
@aryzing could you please provide me with exact name of the type or function you want to use
Seems to be that you should create a PR and export this type. I don' see how it can be used without calling the function.
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