02 — Data files

Move state out of the server file into JSON. Edit the JSON; the endpoint reloads in place.

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/02-data-files/server.ts

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

Concept

dataFile reads a JSON file at boot and on disk change (debounced 100 ms). Shape decides behavior:

• Array JSON → list endpoint, full CRUD.
• Object JSON → record endpoint, GET / PATCH / PUT.

Wrap the path with file<T>('./x.json') to carry the JSON shape into the handle's type without committing to a static import (so JSON edits keep hot-reloading).

Code

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

interface Product { id: number; name: string; price: number; stock: number }
interface AppConfig { feature_dark_mode: boolean; max_upload_mb: number }

type Endpoints = {
  '/api/products': Product[];
  '/api/config': AppConfig;
};

mockr<Endpoints>({
  port: 3002,
  endpoints: [
    { url: '/api/products', dataFile: file<Product[]>('./products.json') },
    { url: '/api/config',   dataFile: file<AppConfig>('./config.json') },
  ],
});

Hot-reload semantics

Editing the file resets the endpoint: in-memory mutations (POSTs since the last reload) are dropped; the file content becomes the new state. Last-known-good is kept across bad JSON, so a typo doesn't crash the server.

Try it

Open in StackBlitz → — paste each curl into the StackBlitz Terminal once npx tsx examples/02-data-files/server.ts is running.

# list (loaded from products.json)
curl -s http://localhost:3002/api/products

# insert — persists in memory, dropped on next file edit
curl -s -X POST http://localhost:3002/api/products \
  -H 'Content-Type: application/json' \
  -d '{"name":"Webcam","price":60,"stock":12}'

# record endpoint, shallow merge
curl -s -X PATCH http://localhost:3002/api/config \
  -H 'Content-Type: application/json' \
  -d '{"max_upload_mb":100}'

Now save products.json with one item removed — GET /api/products reflects it on the next request.

What's next

Join data across endpoints inside a handler → 03 — Cross-endpoint.