Fully typed API client with zod and fetch

api.ts

import { z } from "zod";

const Post = z.object({
  id: z.string(),
  slug: z.string(),
  content: z.string(),
  title: z.string()
})

export function createAPIClient() {
  async function _fetch<T extends z.ZodTypeAny>(
    input: RequestInfo,
    init: RequestInit,
    type: T
  ): Promise<z.infer<T>> {
    const headers = new Headers(init?.headers);

    const res = await fetch(input, { ...init, headers });

    if (!res.ok) {
      throw new Error(res.statusText);
    }
    const data = await res.json();
    const result = await type.safeParseAsync(data);

    if (!result.success) {
      throw new Error(result.error.message);
    }

    return result.data;
  }

  async function getPostsForBlog(blogSlug: string) {
    const res = await _fetch(
      `/api/blogs/${blogSlug}/posts`,
      { method: "GET" },
      z.array(ClientPost)
    );
    return res;
  }

  return {
    posts: {
      get: (slug: string) => _fetch(`/api/posts/${slug}`, { method: "GET" }, Post),
      getAll: _fetch('/api/posts', { method: 'GET' }, z.array(Post))
  };
}