Regression suite

• All my tests pass
• total({"lines": [{"sku": "tea", "priceCents":…) returns 1097
• No uncaught errors

// The code under test: Mangrove Market's basket.
export function addItem(cart, item, qty = 1) {
  if (!Number.isInteger(qty) || qty < 1) throw new Error('qty must be a whole number of at least 1');
  const lines = cart.lines.map((l) => ({ ...l }));
  const line = lines.find((l) => l.sku === item.sku);
  if (line) line.qty += qty; else lines.push({ sku: item.sku, priceCents: item.priceCents, qty });
  return { ...cart, lines };
}

export function removeItem(cart, sku) {
  return { ...cart, lines: cart.lines.filter((l) => l.sku !== sku) };
}

export function total(cart) {
  const sum = cart.lines.reduce((s, l) => s + l.qty * l.priceCents, 0);
  const off = cart.coupon ? Math.min(sum, cart.coupon.offCents) : 0;
  return sum - off;
}

// Test data builders: each test says only what matters to it.
export const tea = { sku: 'tea', priceCents: 399 };
export const rice = { sku: 'rice-5kg', priceCents: 1250 };

export function aCart(overrides = {}) {
  return { lines: [], coupon: null, ...overrides };
}

import { addItem, removeItem, total } from './cart';
import { aCart, tea, rice } from './fixtures';

// Tags in the name: [smoke] runs on every deploy; [bug #id] keeps a fixed bug fixed.
// Group by feature with the name prefix, so a failing run reads like a list of broken features.

test('[smoke] basket › adding an item shows it with its price', () => {
  expect(addItem(aCart(), tea).lines).toEqual([{ sku: 'tea', priceCents: 399, qty: 1 }]);
});
test('[smoke] basket › the total adds every line', () => {
  expect(total(addItem(addItem(aCart(), tea, 2), rice))).toBe(2048);
});
test('basket › adding the same item again raises its quantity', () => {
  expect(addItem(addItem(aCart(), tea), tea, 2).lines[0].qty).toBe(3);
});
test('basket › removing an item removes the whole line', () => {
  expect(removeItem(addItem(aCart(), tea), 'tea').lines).toEqual([]);
});
test('basket › adding never changes the basket it was given', () => {
  const before = addItem(aCart(), tea);
  addItem(before, tea);
  expect(before.lines[0].qty).toBe(1);
});
test('basket › a quantity of 0 is refused', () => {
  expect(() => addItem(aCart(), tea, 0)).toThrow('at least 1');
});
test('[bug #212] coupons › a coupon bigger than the basket makes it free, not negative', () => {
  expect(total(aCart({ lines: [{ sku: 'tea', priceCents: 399, qty: 1 }], coupon: { offCents: 500 } }))).toBe(0);
});
test('[bug #187] coupons › an empty basket with a coupon costs 0', () => {
  expect(total(aCart({ coupon: { offCents: 500 } }))).toBe(0);
});