[SOLVED] Getting the type of applied generic function

Issue

In TypeScript, if I have a function, I can get the type of that function:

function foo(v: number) {}

type FooType = typeof foo; // RESULT: (v: number) => void

If I have a generic function, I can get the type of that function, and the type indicates that it is generic.

function foo<T>(v: T) {}

type FooType = typeof foo; // RESULT: <T>(v: T) => void

How can I get the type of the generic function when a known set of types is used? This doesn’t work.

function foo<T>(v: T) {}

type FooNumberType = typeof foo<number>; // DESIRED: (v: number) => void
                               ^ error TS1005: ';' expected.

Solution

This is not possible in Typescript 4.6 (the current release of as March 23, 2022). You cannot get the type of a generic function as if its generic was set to something.

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However, your desired code works without modification in the unreleased Typescript 4.7.

See this playground which is set to use the nightly 4.7 build of Typescript.

See issue #47607 on "Instantiation Expressions" for more info about this change, which has this canonical example:

function makeBox<T>(value: T) {
    return { value };
};

const makeStringBox = makeBox<string>;  // (value: string) => { value: string }
const stringBox = makeStringBox('abc');  // { value: string }

const ErrorMap = Map<string, Error>;  // new () => Map<string, Error>
const errorMap = new ErrorMap();  // Map<string, Error>

Answered By – Alex Wayne

Answer Checked By – Pedro (BugsFixing Volunteer)