export class HttpError extends Error {
  constructor(
    readonly status: number,
    message: string,
  ) {
    super(message);
    this.name = "HttpError";
  }
}

export async function fetchJson<T>(
  url: string,
  init?: RequestInit,
  guard?: (data: unknown) => data is T,
): Promise<T> {
  const response = await fetch(url, {
    ...init,
    headers: { Accept: "application/json", ...init?.headers },
  });
  if (!response.ok) {
    throw new HttpError(response.status, `HTTP ${response.status} for ${url}`);
  }
  const data: unknown = await response.json();
  if (guard && !guard(data)) {
    throw new Error(`Unexpected JSON shape from ${url}`);
  }
  return data as T;
}

type User = { id: number; email: string; name: string };

function isUser(data: unknown): data is User {
  if (typeof data !== "object" || data === null) return false;
  const u = data as Record<string, unknown>;
  return (
    typeof u.id === "number" &&
    typeof u.email === "string" &&
    typeof u.name === "string"
  );
}

async function main() {
  const user = await fetchJson<User>(
    "https://jsonplaceholder.typicode.com/users/1",
    undefined,
    isUser,
  );
  console.log(user.email);
}

main();