Shopping cart kata using pnpm

test-pnpm/shopping-cart-kata/README.md

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+# What do we want to build?
+
+We are building a shopping cart for an online grocery shop. The idea of this kata is to build the product in an iterative way.
+
+### Technical requirements
+
+- The price per unit is calculated based on the product cost and the percentage of revenue that the company wants for that product.
+- The price has to be rounded up; so if a price per unit calculated is 1.7825, then the expected price per unit for that product is 1.79
+- The final price of the product is then calculated as the **price per unit with the VAT rounded up**.
+- Products are not allowed to have the same name.
+
+### List of products
+
+| **Name**       | **Cost** | **% Revenue** | **Price per unit** | **Tax**                 | **Final price** |
+| -------------- | -------- | ------------- | ------------------ | ----------------------- | --------------- |
+| **Iceberg 🥬** | _1.55 €_ | _15 %_        | 1,79 €             | _Normal (21%)_          | 2.17 €          |
+| **Tomato 🍅**  | _0.52 €_ | _15 %_        | 0.60 €             | _Normal (21%)_          | 0.73 €          |
+| **Chicken 🍗** | _1.34 €_ | _12 %_        | 1.51 €             | _Normal (21%)_          | 1.83 €          |
+| **Bread 🍞**   | _0.71 €_ | _12 %_        | 0.80 €             | _First necessity (10%)_ | 0.89 €          |
+| **Corn 🌽**    | _1.21 €_ | _12 %_        | 1.36 €             | _First necessity (10%)_ | 1.50 €          |
+
+### List of discounts
+
+| **Discounts code** | **Amount** |
+| :----------------: | ---------- |
+|    **PROMO_5**     | 5%         |
+|    **PROMO_10**    | 10%        |
+
+### Use cases
+
+#### List the shopping cart
+
+> ```
+> As a customer
+> I want to see my shopping cart
+> ```
+
+#### **Empty cart**
+
+```
+--------------------------------------------
+| Product name | Price with VAT | Quantity |
+| -----------  | -------------- | -------- |
+|------------------------------------------|
+| Promotion:                               |
+--------------------------------------------
+| Total products: 0                        |
+| Total price: 0.00 €                      |
+--------------------------------------------
+```
+
+#### Add product to shopping cart
+
+> ```
+> As a customer
+> I want to add Iceberg 🥬 to my shopping cart
+> I want to add Tomato  🍅 to my shopping cart
+> I want to add Chicken 🍗 to my shopping cart
+> I want to add Bread 🍞 to my shopping cart
+> I want to add Corn 🌽 to my shopping cart
+> I want to see my shopping cart
+> ```
+
+```
+--------------------------------------------
+| Product name | Price with VAT | Quantity |
+| -----------  | -------------- | -------- |
+| Iceberg 🥬   | 2.17 €         | 1        |
+| Tomato  🍅   | 0.73 €         | 1        |
+| Chicken 🍗   | 1.83 €         | 1        |
+| Bread 🍞     | 0.88 €         | 1        |
+| Corn 🌽      | 1.50 €         | 1        |
+|------------------------------------------|
+| Promotion:                               |
+--------------------------------------------
+| Total products: 5                        |
+| Total price: 7.11 €                      |
+--------------------------------------------
+```
+
+#### Add product to shopping cart
+
+> ```
+> As a customer
+> I want to add Iceberg 🥬 to my shopping cart
+> I want to add Iceberg 🥬 to my shopping cart
+> I want to add Iceberg 🥬 to my shopping cart
+> I want to add Tomato  🍅 to my shopping cart
+> I want to add Chicken 🍗 to my shopping cart
+> I want to add Bread 🍞 to my shopping cart
+> I want to add Bread 🍞 to my shopping cart
+> I want to add Corn 🌽 to my shopping cart
+> I want to see my shopping cart
+> ```
+
+```
+--------------------------------------------
+| Product name | Price with VAT | Quantity |
+| -----------  | -------------- | -------- |
+| Iceberg 🥬   | 2.17 €         | 3        |
+| Tomato  🍅   | 0.73 €         | 1        |
+| Chicken 🍗   | 1.83 €         | 1        |
+| Bread 🍞     | 0.88 €         | 2        |
+| Corn 🌽      | 1.50 €         | 1        |
+|------------------------------------------|
+| Promotion:                               |
+--------------------------------------------
+| Total products: 8                        |
+| Total price: 12.33 €                     |
+--------------------------------------------
+```
+
+#### Apply discount to the shopping cart
+
+> ```
+> As a customer
+> I want to add Iceberg 🥬 to my shopping cart
+> I want to add Iceberg 🥬 to my shopping cart
+> I want to add Iceberg 🥬 to my shopping cart
+> I want to add Tomato  🍅 to my shopping cart
+> I want to add Chicken 🍗 to my shopping cart
+> I want to add Bread 🍞 to my shopping cart
+> I want to add Bread 🍞 to my shopping cart
+> I want to add Corn 🌽 to my shopping cart
+> I want to apply my coupon code PROMO_5
+> I want to see my shopping cart
+> ```
+
+```
+--------------------------------------------
+| Product name | Price with VAT | Quantity |
+| -----------  | -------------- | -------- |
+| Iceberg 🥬   | 2.17 €         | 3        |
+| Tomato  🍅   | 0.73 €         | 1        |
+| Chicken 🍗   | 1.83 €         | 1        |
+| Bread 🍞     | 0.88 €         | 2        |
+| Corn 🌽      | 1.50 €         | 1        |
+|------------------------------------------|
+| Promotion: 5% off with code PROMO_5      |
+--------------------------------------------
+| Total products: 8                        |
+| Total price: 11.71 €                     |
+--------------------------------------------
+```
+
+### Possible API for the ShoppingCart
+
+**Approach 1 passing objects as arguments could be DTO**
+
+```javascript
+export interface ShoppingCart {
+  addItem(cartItem: CartItem): void;
+  applyDiscount(discount: Discount): void;
+  printShoppingCart(): void;
+  // TODO
+  removeItem(name: string): void;
+  updateItemQuantity(name: string, quantity: number): void;
+  getItems(): ReadonlyArray<CartItem>;
+}
+```
+
+### Summary of steps
+
+**Inside-Out (classicist)** vs **Outside-In (London School / mockist)** has significant implications for how the system grows. Since I am aiming for **small, logical, test-driven steps**, I will start inside out.
+
+##### Suggested Inside-Out Iterations (with TDD focus)
+
+| Iteration | Focus                                    | Why Start Here?                                              |
+| --------- | ---------------------------------------- | ------------------------------------------------------------ |
+| 1         | `Product` - price per unit and VAT logic | Small, atomic, pure — perfect TDD start                      |
+| 2         | `CartItem` - quantity & total price      | Introduces aggregation, still isolated logic                 |
+| 3         | `Discount` - percentage logic            | Stateless, predictable, complements pricing                  |
+| 4         | `ShoppingCart` - add items               | Start using composition of domain elements                   |
+| 5         | `ShoppingCart` - apply discount          | Introduces first mutation to cart state                      |
+| 6         | `ShoppingCart` - total price & print     | Drives end-to-end expectation-based testing                  |
+| 7         | `ProductCatalogue` or Registry           | Encapsulate product lookup and validation                    |
+| 8         | `ShoppingCart` - delete items            | TODO - Drive removing Items from the cart                    |
+| 9         | `ShoppingCart` - updateItemQuantity      | TODO - Drive changing the quantity                           |
+| 10        | `ShoppingCart` - getItems                | TODO - Drive getting the items to be more inline with a cart |
+
+### Evolution Flow Summary
+
+#### Phase 1: Pure Domain Logic
+
+- `Product` — validate markup/VAT/rounding rules
+- `CartItem` — quantity x price logic
+- `Discount` — test discount % and edge cases (e.g. invalid codes)
+
+#### Phase 2: Composition
+
+- `ShoppingCart` — manages cart items and uses the above types
+- Discounts applied as decorators or cart state
+
+#### Phase 3: Orchestration + I/O-style Outputs
+
+- Implement `ShoppingCart.getShoppingCart()` as DTO for rendering
+- `print()` or `toString()` as test-driving format rendering
+- Possibly inject or access product catalogue here
+
+### When to Consider _Outside-In_
+
+You could start Outside-In **only** if:
+
+- You’re driving everything from the end goal (like rendering the full cart view)
+- You're willing to mock deeper domain types (`Product`, `CartItem`, etc.)
+
+But in this kata, that’s harder to justify because the logic in the "leaf" types is richer and more test-worthy than in the orchestration layer.
+
+### Summary
+
+Thanks Emmanuael Valverde, for your cool take on this [kata](https://www.codurance.com/katas/shopping-cart-kata). There were a few sneaky catches with the pricing. I have done this kata before using simple types and I found myself producing more code this time. I also did outside in, before, mocking and faking bits until I had the final design. I think breaking this up into the seperate phases made it easier to get my head around each concept, so I am glad I did it this way. The thing didn't like is it forced me into a solution by virtue of what I simplified. It may be a good or bad thing, no idea.

test-pnpm/shopping-cart-kata/cartItem.ts

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+import { Product } from './product';
+import { roundUp } from './utility';
+
+export class CartItem {
+  constructor(
+    public product: Product,
+    public quantity: number,
+  ) {}
+
+  get LineTotal(): number {
+    const price = this.product.finalPrice * this.quantity;
+    return roundUp(price);
+  }
+}

test-pnpm/shopping-cart-kata/cartitem.should.test.ts

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+import { CartItem } from './cartItem';
+import { Product } from './product';
+import { TaxRate } from './taxRates';
+
+describe('Adding Cart items should', () => {
+  let product = new Product('Iceberg 🥬', 1.55, 0.15, TaxRate.Normal);
+
+  test('calcultate the line total for one item', () => {
+    const cartItem = new CartItem(product, 1);
+
+    expect(cartItem.LineTotal).toBe(2.17);
+  });
+
+  test('calcultate the line total for multiple items', () => {
+    const cartItem = new CartItem(product, 3);
+
+    expect(cartItem.LineTotal).toBe(6.51);
+  });
+});

test-pnpm/shopping-cart-kata/constants.ts

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+
+export const TABLE_CONSTANTS = {
+  LINE_WIDTH: 44,
+  COLUMN_WIDTHS: {
+    NAME: 12,
+    PRICE: 14,
+    QUANTITY: 8,
+  },
+  PADDING_WIDTH: 40,
+  CURRENCY: '€',
+  SEPARATORS: {
+    HORIZONTAL: '-',
+    VERTICAL: '|',
+  },
+};

test-pnpm/shopping-cart-kata/discount.should.test.ts

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+import { Discount } from './discount';
+
+describe('Discount should', () => {
+  it('be created from a valid discount code', () => {
+    const discount = Discount.fromCode('PROMO_5');
+
+    expect(discount.code).toBe('PROMO_5');
+    expect(discount.percentage).toBe(0.05);
+  });
+
+  it('return null for an unknown code', () => {
+    const discount = Discount.fromCode('INVALID');
+
+    expect(discount).toBeNull();
+  });
+
+  it('calculate a discount from a price', () => {
+    const discount = Discount.fromCode('PROMO_10')!;
+    expect(discount.applyTo(100)).toBe(90);
+  });
+});

test-pnpm/shopping-cart-kata/discount.ts

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+import { roundUp } from './utility';
+
+export class Discount {
+  readonly code: string;
+  readonly percentage: number;
+
+  private constructor(code: string, percentage: number) {
+    this.code = code;
+    this.percentage = percentage;
+  }
+
+  public applyTo(price: number): number {
+    const discountAmount = price * this.percentage;
+    const discountedPrice = price - discountAmount;
+    return roundUp(discountedPrice);
+  }
+
+  static fromCode(code: string): Discount | null {
+    const map: Record<string, number> = {
+      PROMO_5: 0.05,
+      PROMO_10: 0.1,
+    };
+    const value = map[code];
+    return value ? new Discount(code, value) : null;
+  }
+}

test-pnpm/shopping-cart-kata/logger.ts

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+export interface Logger {
+  log(message: string): void;
+  print(): string;
+  clear(): void;
+}
+
+export class InMemoryLogger implements Logger {
+  private messages: string[];
+
+  constructor() {
+    this.clear();
+  }
+
+  clear(): void {
+    this.messages = [];
+  }
+
+  log(message: string): void {
+    this.messages.push(message);
+  }
+
+  print(): string {
+    return this.messages.join('\n');
+  }
+}