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Shopping Cart

A complete shopping cart implementation with add/remove items, quantity updates, and dynamic total calculations. Demonstrates reactive data management and user interaction patterns.

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Features Demonstrated

  • Array manipulation - Add, remove, update cart items

  • Lambda expressions - Complex calculations directly in templates using reduce, filter, etc.

  • Event handling - Button clicks, quantity changes

  • Conditional rendering - Empty cart state, checkout button

  • List rendering with keys - Efficient cart item updates

  • Two-way binding - Quantity inputs

  • Number formatting - Currency display

  • Component state management - Cart as a service

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Code

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View Model (shopping-cart.ts)

export interface CartItem {
  id: number;
  productId: number;
  name: string;
  price: number;
  quantity: number;
  image: string;
  maxQuantity: number;
}

export class ShoppingCart {
  cartItems: CartItem[] = [];

  // Add item to cart
  addToCart(product: { id: number; name: string; price: number; image: string; maxQuantity: number }) {
    const existingItem = this.cartItems.find(item => item.productId === product.id);

    if (existingItem) {
      // Increase quantity if item already in cart
      if (existingItem.quantity < existingItem.maxQuantity) {
        existingItem.quantity++;
      } else {
        alert(`Maximum quantity (${existingItem.maxQuantity}) reached for ${existingItem.name}`);
      }
    } else {
      // Add new item
      this.cartItems.push({
        id: Date.now(), // Simple ID generation
        productId: product.id,
        name: product.name,
        price: product.price,
        quantity: 1,
        image: product.image,
        maxQuantity: product.maxQuantity
      });
    }
  }

  // Update item quantity
  updateQuantity(item: CartItem, newQuantity: number) {
    if (newQuantity <= 0) {
      this.removeItem(item);
    } else if (newQuantity <= item.maxQuantity) {
      item.quantity = newQuantity;
    } else {
      item.quantity = item.maxQuantity;
      alert(`Maximum quantity is ${item.maxQuantity}`);
    }
  }

  // Increase quantity
  increaseQuantity(item: CartItem) {
    if (item.quantity < item.maxQuantity) {
      item.quantity++;
    } else {
      alert(`Maximum quantity (${item.maxQuantity}) reached`);
    }
  }

  // Decrease quantity
  decreaseQuantity(item: CartItem) {
    if (item.quantity > 1) {
      item.quantity--;
    } else {
      this.removeItem(item);
    }
  }

  // Remove item from cart
  removeItem(item: CartItem) {
    const index = this.cartItems.indexOf(item);
    if (index > -1) {
      this.cartItems.splice(index, 1);
    }
  }

  // Clear entire cart
  clearCart() {
    if (confirm('Are you sure you want to clear your cart?')) {
      this.cartItems = [];
    }
  }

  // Proceed to checkout
  checkout() {
    console.log('Proceeding to checkout with:', this.cartItems);
    alert('Proceeding to checkout...');
    // In a real app, navigate to checkout page or open checkout modal
  }
}

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Currency Value Converter (currency-value-converter.ts)

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Template (shopping-cart.html)

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Styles (shopping-cart.css)

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How It Works

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1. Lambda Expressions in Templates

Instead of computed properties in the view model, calculations are done directly in the template using lambda expressions:

Aurelia's lambda expressions support complex operations like reduce, filter, map, every, and some directly in templates. The template automatically tracks dependencies and recalculates when cartItems changes.

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2. Array Manipulation

Using array methods ensures change detection:

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3. Benefits of Lambda Expressions

Moving calculations to the template has several advantages:

  • Reduced boilerplate - No need for getter methods in the view model

  • Clear intent - Calculations are visible right where they're used

  • Single source of truth - The template directly expresses what data it needs

  • Automatic reactivity - Aurelia tracks all dependencies within lambda expressions

This approach is particularly useful for derived data that's only needed in the view.

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4. Efficient List Updates

Using key: id allows Aurelia to track items efficiently:

When items are removed or reordered, Aurelia reuses DOM elements.

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5. Quantity Validation

Multiple ways to update quantity with validation:

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6. Conditional Rendering

Show different UI based on cart state:

Lambda expressions work seamlessly with conditionals: if.bind="cartItems.length" or if.bind="!cartItems.length".

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7. Currency Formatting with Value Converter

The currency value converter formats prices consistently:

The converter uses Intl.NumberFormat for proper currency formatting including the currency symbol, decimal places, and thousands separators. This keeps formatting logic out of the view model.

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8. When to Use Lambda Expressions vs Computed Properties

Use lambda expressions in templates when:

  • The calculation is only needed in the view

  • The logic is straightforward and readable inline

  • You want to reduce view model boilerplate

Use computed properties in the view model when:

  • The calculation is complex and would make the template hard to read

  • The value is used in multiple places (template and view model logic)

  • You need to unit test the calculation logic

  • The calculation is expensive and you want explicit memoization

For this shopping cart example, the calculations are simple arithmetic operations that are only displayed to the user, making lambda expressions a great fit. They eliminate boilerplate while keeping the template clear and maintainable.

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Variations

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Persist Cart to LocalStorage

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Add Discount Codes

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Cart as a Service

Make the cart available throughout the app:

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Related

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