03 — Cross-endpoint joins

Inside a handler, ctx.endpoint(url) gives a typed handle to any other endpoint's data.

Run this chapter in 30 seconds

1. Open in StackBlitz → — full Node sandbox in your browser, no install.
2. Wait for npm install to finish, then in the Terminal tab run:

   npx tsx examples/03-cross-endpoint/server.ts

3. Paste any request from the Try it section below into the Terminal (use curl — the StackBlitz preview port is forwarded).

Concept

Convention: /internal/* endpoints hold source-of-truth data. /api/* endpoints are thin handlers that join, filter, or mutate them. The frontend never calls /internal/* directly.

ctx.endpoint('/internal/products') returns a ListHandle<Product> (or RecordHandle<T> for object data) — typed against the Endpoints map.

Code

import { mockr, handler } from '@yoyo-org/mockr';

interface Product { id: number; name: string; price: number; stock: number }
interface CartItem { id: number; product_id: number; quantity: number }

type Endpoints = {
  '/internal/products': Product[];
  '/internal/cart': CartItem[];
};

mockr<Endpoints>({
  port: 3003,
  endpoints: [
    { url: '/internal/products', data: [/* ... */] },
    { url: '/internal/cart', data: [] },

    {
      url: '/api/cart',
      method: 'POST',
      handler: handler({ fn: (req, ctx) => {
        const { product_id, quantity } = req.body as { product_id: number; quantity: number };
        const products = ctx.endpoint('/internal/products');
        const cart = ctx.endpoint('/internal/cart');

        const product = products.findById(product_id);
        if (!product) return { status: 404, body: { error: 'not found' } };
        if (product.stock < quantity) return { status: 400, body: { error: 'out of stock' } };

        products.update(product_id, { stock: product.stock - quantity });
        const item = cart.insert({ product_id, quantity } as CartItem);
        return { status: 201, body: { item } };
      } }),
    },
  ],
});

Try it

Open in StackBlitz → — paste each curl into the StackBlitz Terminal once npx tsx examples/03-cross-endpoint/server.ts is running.

# empty cart
curl -s http://localhost:3003/api/cart

# add a line — decrements /internal/products stock atomically
curl -s -X POST http://localhost:3003/api/cart \
  -H 'Content-Type: application/json' \
  -d '{"product_id":1,"quantity":2}'

# cart now joined with products + computed total
curl -s http://localhost:3003/api/cart

The first GET joins cart × products and computes a total. The POST decrements stock on /internal/products and inserts into /internal/cart — atomic from the client's perspective.

What's next

Validate request body / query / params with zod schemas → 04 — Handlers + zod.