TypeScriptsrc/cart.test.ts
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import { describe, it, expect } from 'vitest';
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import { ShoppingCart } from './cart';
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describe('ShoppingCart — TDD', () => {
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  // Red 1: empty cart
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  it('starts empty with zero total', () => {
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    const cart = new ShoppingCart();
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    expect(cart.itemCount).toBe(0);
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    expect(cart.total).toBe(0);
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  });
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  // Red 2: add item
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  it('adds item and updates total', () => {
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    const cart = new ShoppingCart();
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    cart.add({ id: '1', name: 'Book', price: 29.99, qty: 2 });
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    expect(cart.itemCount).toBe(1);
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    expect(cart.total).toBeCloseTo(59.98);
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  });
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  // Red 3: remove item
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  it('removes item by id', () => {
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    const cart = new ShoppingCart();
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    cart.add({ id: '1', name: 'Book', price: 10, qty: 1 });
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    cart.add({ id: '2', name: 'Pen', price: 2, qty: 3 });
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    cart.remove('1');
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    expect(cart.itemCount).toBe(1);
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    expect(cart.total).toBeCloseTo(6);
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  });
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  // Red 4: discount
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  it('applies percentage discount code', () => {
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    const cart = new ShoppingCart();
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    cart.add({ id: '1', name: 'Shirt', price: 100, qty: 1 });
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    cart.applyDiscount('SAVE20'); // 20% off
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    expect(cart.total).toBeCloseTo(80);
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  });
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  // Red 5: invalid discount
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  it('ignores unknown discount codes', () => {
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    const cart = new ShoppingCart();
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    cart.add({ id: '1', name: 'Hat', price: 30, qty: 1 });
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    cart.applyDiscount('INVALID');
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    expect(cart.total).toBeCloseTo(30);
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  });
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});