const functions, overloads and generics typescript

Question

Typescript supports function overloading, which is pretty cool, and it's possible to make a constant function overloaded like this:

interface FetchOverload {
  (action: string, method: 'post' | 'get'): object;
  (action: string): object
}

const fetch: FetchOverload = (action: string, method: 'get' | 'post' = 'get'): object { ... }

But let's suppose I want to inform the type of response expected , instead of any object . I can do this by simply replacing object with the desired type:

interface FetchOverload {
  (action: string, method: 'post' | 'get'): UserResponse;
  (action: string): UserResponse
}

const fetch: FetchOverload = (action: string, method: 'get' | 'post' = 'get'): UserResponse { ... }

If I want to go even further and instead of saying it can only return a response of type UserResponse , I could return a generic type:

interface FetchOverload<T> {
  (action: string, method: 'post' | 'get'): T;
  (action: string): T
}

const fetch: FetchOverload<T> = (action: string, method: 'get' | 'post' = 'get'): T { ... }

The problem is that for const there is no a generic type T . And in fact there isn't, I need to declare somewhere that T is a generic type, but I don't know where!

I tried everywhere and couldn't make it understand that there is a type T :

<T> const fetch: ...
const <T> fetch: ...
const fetch <T>: ...
const fetch: <T> FetchOverload<T> = ...
const fetch: FetchOverload<T> = <T> (action: ...)

The only solution I found was to transform the const function into native functions with overload:

function fetch<T>(action: string, method: 'post' | 'get'): T;
function fetch<T>(action: string): T;
function fetch<T>(action: string, method: 'post' | 'get' = 'get'): T { ... }

So what I would like to know is if in fact I need to use it this way, or if there is still some solution to keep using const .

Answer 1

I "discovered" the answer as I ended up encountering a very similar problem in this question , this question and in this answer and this answer . Basically I would just do:

interface FetchOverload {
  <T> (action: string, method: 'post' | 'get'): T;
  <T> (action: string): T
}

export const fetch: FetchOverload = <T,> (action: string, method: 'get' | 'post' = 'get'): T { ... }

Actually, the only weird thing is that comma after the type, but it's mandatory, so I don't have much to do. But it's all settled!