Get a typed JSON from any API

json.ts

import { go, no, type Result } from "./result.ts";

/**
 * Fetch JSON data and parse it safely.
 */
export const fetchJSON = async <T>(
  url: Parameters<typeof fetch>[0],
  {
    headers,
    parser,
  }: {
    headers?: HeadersInit;
    parser: (data: unknown) => T;
  },
): Promise<Result<"InvalidData" | "NetworkError", T>> => {
  const response = await fetch(url, { headers })
    .catch(() => no("NetworkError"));

  if (!response.ok) {
    return no("NetworkError");
  }

  return response.json()
    .then((data) => parser(data))
    .then((valid) => go(valid))
    .catch(() => no("InvalidData"));
};

pokeapi.ts

import { number, object, parse } from "jsr:@valibot/valibot";
import { fetchJSON } from "./json.ts";

const schema = object({
  base_experience: number(),
});

if (import.meta.main) {
  // invoke like so:
  // deno run -A pokeapi.ts charmander
  const [pokemon] = Deno.args;
  const result = await fetchJSON(
    `https://pokeapi.co/api/v2/pokemon/${pokemon}`,
    {
      parser: (data) => parse(schema, data),
    },
  );

  if (!result.ok) {
    console.error(result.no);
  } else {
    console.log(result.go.base_experience);
  }
}

result.ts

export type Result<No, Go> = { ok: false; no: No } | { ok: true; go: Go };

export function no<const T>(no: T): { ok: false; no: T } {
  return { ok: false, no };
}

export function go<const T>(go: T): { ok: true; go: T } {
  return { ok: true, go };
}