Shipping tests that only confirm the code

The checkout service needs a shipping fee calculator. A teammate gave the AI assistant the pricing rules from the product spec and asked for "`calculateShipping(order)` with tests". Both the function and its tests came back in one go, and the test run is green. You are reviewing **the function and its tests together**. The visible tests are the ones the assistant wrote; treat them as part of the code under review, not as an independent oracle. The pricing rules from the spec: - A standard order whose subtotal is **100 or more** ships free. - Otherwise the fee is a base rate of 5.99, plus 1.50 for **every started kilogram above 2 kg** (2.0 kg adds nothing, 2.1 kg adds one kilogram, 3.0 kg adds one, 3.01 kg adds two). - Express delivery adds 9.99 on top of whatever the standard fee would be, so a free-shipping order pays 9.99 for express. - The result is rounded to two decimals. Decide whether the tests actually prove the rules, and whether the implementation follows them.

import { describe, it, expect } from "@debunk/harness";
import {
  calculateShipping,
  BASE_RATE,
  PER_EXTRA_KG,
  EXPRESS_SUPPLEMENT,
} from "./code";

describe("calculateShipping", () => {
  it("ships free above the threshold", () => {
    expect(calculateShipping({ subtotal: 150, weightKg: 1 })).toBe(0);
  });

  it("charges the base rate plus the per-kg surcharge", () => {
    expect(calculateShipping({ subtotal: 40, weightKg: 1 })).toBe(BASE_RATE);
    expect(calculateShipping({ subtotal: 40, weightKg: 4 })).toBe(BASE_RATE + 2 * PER_EXTRA_KG);
  });

  it("adds the express supplement", () => {
    expect(calculateShipping({ subtotal: 40, weightKg: 1, express: true })).toBe(
      BASE_RATE + EXPRESS_SUPPLEMENT,
    );
  });
});