Task: Design Test Doubles for a Repository Interface

Given the following repository interface and use case, produce the three standard test doubles: a Stub, a Mock, and a Fake.

Interface and Use Case

// src/ports/IProductRepository.ts
export interface Product {
  id: string
  name: string
  price: number
  stock: number
}

export interface IProductRepository {
  findById(id: string): Promise<Product | null>
  save(product: Product): Promise<void>
  findAll(): Promise<Product[]>
}

// src/use-cases/ReserveProductUseCase.ts
export class ReserveProductUseCase {
  constructor(private repo: IProductRepository) {}

  async execute(productId: string, quantity: number): Promise<void> {
    const product = await this.repo.findById(productId)
    if (!product) throw new Error('Product not found')
    if (product.stock < quantity) throw new Error('Insufficient stock')
    product.stock -= quantity
    await this.repo.save(product)
  }
}

Output Specification

Produce a single file test-doubles.ts that contains and exports all three implementations:

1. StubProductRepository — returns fixed product data from findById (a product with id "prod-1", name "Widget", price 9.99, stock 10). save and findAll are no-ops.
2. MockProductRepository — records whether save was called and with what product. Exposes a savedProduct property (initially null). findById returns a product with stock 5.
3. InMemoryProductRepository — a full in-memory implementation that stores products in a Map. All three methods must work correctly.

All three classes must implement IProductRepository.