simple-mock-api.ts

type Method = "GET" | "POST" | "PUT" | "DELETE";
type RouteCallback = (request?: MockRequest) => unknown;

interface Route {
  method: Method;
  url: string;
  callback: RouteCallback;
}

type MockRequest = {
  params?: Record<string, string>;
  method?: Method;
  body?: any;
};

interface MockAPIConfig {
  delay?: number;
  routes: Array<Route>;
}

type FetchFunction = (url: string, request?: MockRequest) => Promise<unknown>;

const methodFunction =
  (method: Method) =>
  (url: string, callback: RouteCallback): Route => ({
    method: method,
    url,
    callback,
  });

const rest = {
  get: methodFunction("GET"),
  post: methodFunction("POST"),
};

function createMockAPI(config: MockAPIConfig): FetchFunction {
  return async function (url: string, request?: MockRequest) {
    await new Promise(r => setTimeout(r, config?.delay ?? 1000));

    const method = request?.method || "GET";
    const route = config.routes.find(
      route => route.url === url && route.method === method,
    );
    return route?.callback(request);
  };
}

const DATA = {
  items: [
    { id: 1, name: "Item 1" },
    { id: 2, name: "Item 2" },
  ],
};

let newId = 3;

const mockFetch = createMockAPI({
  delay: 500,
  routes: [
    rest.get("/items", () => DATA.items),

    rest.get("/items/:id", request => {
      const itemId = request?.params?.id;
      const item = DATA.items.find(item => item.id === Number(itemId));
      return item;
    }),

    rest.post("/items", request => {
      const newItem = {
        ...request!.body,
        id: newId++,
      };

      DATA.items = [...DATA.items, newItem];
    }),
  ],
});

const doTheJob = async () => {
  const a = await mockFetch("/items");
  console.log(a);

  const b = await mockFetch("/items/:id", { params: { id: '1' } });
  console.log(b);

  await mockFetch("/items", { method: "POST", body: { name: "Item X" } });

  const c = await mockFetch("/items");
  console.log(c);
};

doTheJob();