Redux-saga + Typescript: implementing typeguard for Axios AJAX requests

redux-saga-typeguard.ts

import { AxiosResponse } from "axios";

// the kind of data I expect from the AJAX request...
interface AuthData {
  access_token?: string;
  refresh_token?: string;
}
// ... a type dedicated to the Axios response...
type AuthResponse = AxiosResponse<AuthData>;

// ... and the function that consumes Axios... (it's a part of my own function but the rest of the
// body isn't helpful for the sake of the Gist)
function postRefresh(refreshToken: string): Promise<AuthResponse> {
  return new Promise(async (resolve) => {    
    const response: AuthResponse = response = await Axios.post(/* ... */);
    resolve(response);
  });
}

// the saga that consumes the AJAX request
function* UNSAFE_refreshAuthSaga() {
  // ...
  
  // Typescript can't check the return type of yielded value. If 'postRefresh' returns a different 
  // type of response Typescript can't check it and compiles without errors
  const response: AuthResponse = yield call(postRefresh, "the_refresh_token");
  
  // ...
}


function* SAFE_refreshAuthSaga() {
  // ...
  
  // That's a strong typeguard check, useful to work around the Typescript impossibility of checking
  // a yielded function return value
  // tslint:disable-next-line
  let typeguard: Equals<Promise<AuthResponse>, ReturnType<typeof postRefresh>>;
  const response: PromiseArgType<typeof typeguard> = yield call(postRefresh, "the_refresh_token");

  // now if I change the return type of the "postRefresh" function Typescript raises an error

  // ...
}



// some more Typescript utilities
type Equals<A, B> = [A] extends [B] ? ([B] extends [A] ? A : never) : never;

// @see https://stackoverflow.com/questions/48011353/how-to-unwrap-type-of-a-promise?rq=1
type PromiseArgType<T> = T extends Promise<infer U>
  ? U
  : T extends (...args: any[]) => Promise<infer UU>
  ? UU
  : T;