Determining the type of a utilized generic function

Question

Determining the type of a utilized generic function

When working with TypeScript, it is possible to determine the type of a function by using the following method:

function exampleFunc(param: number) {}

type ExampleFuncType = typeof exampleFunc; // RESULT: (param: number) => void

If the function is generic, the type will indicate that it is generic like so:

function genericFunc<T>(param: T) {}

type GenericFuncType = typeof genericFunc; // RESULT: <T>(param: T) => void

However, determining the type of a generic function when a known set of types is used can be tricky:

function genericFunc<T>(param: T) {}

type NumberFuncType = typeof genericFunc<number>; // DESIRED: (param: number) => void
                               ^ error TS1005: ';' expected.

typescript

Answer 1

Answer №1

In the current release of Typescript 4.6 as of March 23, 2022, it is not possible to obtain the type of a generic function as if its generic was set to something.

However, in the upcoming unreleased version of Typescript 4.7, your desired code will work without any modifications.

For a demonstration, you can check out this playground which uses the nightly 4.7 build of Typescript.

Refer to issue #47607 on "Instantiation Expressions" for more details about this change, with a canonical example provided in the code snippet below:

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>

Answer 2

In the current release of Typescript 4.6 as of March 23, 2022, it is not possible to obtain the type of a generic function as if its generic was set to something.

However, in the upcoming unreleased version of Typescript 4.7, your desired code will work without any modifications.

For a demonstration, you can check out this playground which uses the nightly 4.7 build of Typescript.

Refer to issue #47607 on "Instantiation Expressions" for more details about this change, with a canonical example provided in the code snippet below:

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>

Determining the type of a utilized generic function

Answer №1

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