Real World Example

Front ends at Flipkart, Razorpay or any SaaS company wrap HTTP calls in a small typed client. One generic method serves every endpoint, and each call site gets the exact response type. (Network is simulated here.)

Example

type Product = { id: number; name: string; price: number };
type Order = { id: string; total: number; status: "PLACED" | "SHIPPED" };

interface Endpoints {
  "/products": Product[];
  "/orders/latest": Order;
}

const fakeServer: { [K in keyof Endpoints]: Endpoints[K] } = {
  "/products": [{ id: 1, name: "Kettle", price: 1299 }, { id: 2, name: "Toaster", price: 1899 }],
  "/orders/latest": { id: "OD-77", total: 3198, status: "SHIPPED" },
};

class ApiClient {
  private calls = 0;
  async get<P extends keyof Endpoints>(path: P): Promise<Endpoints[P]> {
    this.calls++;
    return structuredClone(fakeServer[path]);
  }
  get callCount(): number { return this.calls; }
}

async function main() {
  const api = new ApiClient();
  const products = await api.get("/products");      // Product[]
  const order = await api.get("/orders/latest");    // Order
  console.log(products.map((p) => p.name).join(", "));
  console.log(order.id, order.status, order.total);
  console.log("calls:", api.callCount);
}
main();

Output

Kettle, Toaster
OD-77 SHIPPED 3198
calls: 2

Note: Mistyping the path (api.get("/product")) is a compile error, and the editor autocompletes the available endpoints.

A Typed API Client

Example