Generic fetch for TypeScript.md

If you wished that you had the ability of declaring the response body's types for your fetch calls beforehand (where you made the call), you can use the following fetch wrapper:

interface GenericResponse<T> extends Response {
  clone(): GenericResponse<T>;
  json(): Promise<T>;
}

export default async function genericFetch<T>(
  input: RequestInfo | URL,
  init?: RequestInit
) {
  return fetch(input, init) as Promise<GenericResponse<T>>;
}

Inspired by ts-generic-fetch.