How backends work: HTTP, REST, JSON and status codes

 Phone app / website (client)                     Server (backend)
 ─────────────────────────────                     ───────────────────────────────
  GET /api/products  ───────── HTTPS request ────►  check token → query database
                     ◄──────── HTTPS response ────  200 OK + JSON list of products

// What a server sends back (as text)
const responseText = `{
  "orders": [
    {"id": 1042, "customer": "Wanjiku", "total": 1240, "paid": true},
    {"id": 1043, "customer": "Otieno", "total": 560, "paid": false}
  ],
  "page": 1,
  "total_pages": 3
}`;

const data = JSON.parse(responseText);            // text -> JavaScript objects
const unpaid = data.orders.filter(o => !o.paid);
console.log("Unpaid orders:", unpaid.map(o => o.id));
console.log("Total value:", data.orders.reduce((s, o) => s + o.total, 0));

const newOrder = { customer: "Amina", items: [{ productId: 17, qty: 2 }] };
console.log(JSON.stringify(newOrder));             // objects -> text to send in a request

// A tiny simulated backend: route the request, check auth, validate, respond
const db = { products: [{ id: 1, name: "Unga 2kg", price: 180 }, { id: 2, name: "Sugar 1kg", price: 150 }] };
const tokens = { "secret-token-123": { userId: 7, role: "owner" } };

function handle(req) {
  const user = tokens[(req.headers.Authorization || "").replace("Bearer ", "")];
  if (!user) return { status: 401, body: { error: "Please sign in" } };

  if (req.method === "GET" && req.path === "/products") {
    return { status: 200, body: db.products };
  }
  if (req.method === "POST" && req.path === "/products") {
    if (user.role !== "owner") return { status: 403, body: { error: "Only owners can add products" } };
    const { name, price } = req.body || {};
    if (!name || typeof price !== "number" || price <= 0) return { status: 422, body: { error: "Name and a positive price are required" } };
    const product = { id: db.products.length + 1, name, price };
    db.products.push(product);
    return { status: 201, body: product };
  }
  return { status: 404, body: { error: "Not found" } };
}

const auth = { Authorization: "Bearer secret-token-123" };
console.log(handle({ method: "GET", path: "/products", headers: {} }));
console.log(handle({ method: "POST", path: "/products", headers: auth, body: { name: "Milk", price: -5 } }));
console.log(handle({ method: "POST", path: "/products", headers: auth, body: { name: "Milk 500ml", price: 60 } }));
console.log(handle({ method: "GET", path: "/orders", headers: auth }));