Building E-commerce Applications with React: A Practical Guide

/*
 * Minimal e-commerce business logic.
 * Run with: node ecommerce.js
 * This example uses in-memory data and no external packages.
 */

console.log("STEP 1: Initialize the product catalog");

const products = [
  { id: "p101", name: "Wireless Headphones", price: 2499, stock: 8 },
  { id: "p102", name: "Mechanical Keyboard", price: 3999, stock: 5 },
  { id: "p103", name: "USB-C Hub", price: 1499, stock: 12 }
];

// Keep cart entries small: product ID and selected quantity.
const cart = new Map();

function findProduct(productId) {
  return products.find((product) => product.id === productId);
}

console.log("Available products:");
products.forEach((product) => {
  console.log(`- ${product.name}: ₹${product.price}`);
});

console.log("\nSTEP 2: Add products to the cart");

function addToCart(productId, quantity = 1) {
  const product = findProduct(productId);

  if (!product) {
    throw new Error(`Product ${productId} does not exist.`);
  }

  if (!Number.isInteger(quantity) || quantity < 1) {
    throw new Error("Quantity must be a positive integer.");
  }

  const currentQuantity = cart.get(productId) || 0;
  const requestedQuantity = currentQuantity + quantity;

  // Validate the requested quantity against available stock.
  if (requestedQuantity > product.stock) {
    throw new Error(`Only ${product.stock} units of ${product.name} are available.`);
  }

  cart.set(productId, requestedQuantity);
  console.log(`Added ${quantity} x ${product.name} to the cart.`);
}

addToCart("p101", 2);
addToCart("p103", 1);

console.log("\nSTEP 3: Update a cart quantity");

function updateQuantity(productId, quantity) {
  if (!cart.has(productId)) {
    throw new Error("This product is not in the cart.");
  }

  if (!Number.isInteger(quantity) || quantity < 0) {
    throw new Error("Quantity must be a non-negative integer.");
  }

  const product = findProduct(productId);

  if (quantity > product.stock) {
    throw new Error("Requested quantity exceeds available inventory.");
  }

  // A quantity of zero removes the product entirely.
  if (quantity === 0) {
    cart.delete(productId);
    console.log(`Removed ${product.name} from the cart.`);
    return;
  }

  cart.set(productId, quantity);
  console.log(`Updated ${product.name} quantity to ${quantity}.`);
}

updateQuantity("p103", 2);

console.log("\nSTEP 4: Calculate the subtotal");

function calculateSubtotal() {
  let subtotal = 0;

  for (const [productId, quantity] of cart.entries()) {
    const product = findProduct(productId);
    subtotal += product.price * quantity;
  }

  return subtotal;
}

const subtotal = calculateSubtotal();
console.log(`Cart subtotal: ₹${subtotal.toFixed(2)}`);

console.log("\nSTEP 5: Apply a discount code");

function calculateDiscount(subtotal, code) {
  const discountCodes = {
    SAVE10: { percentage: 10, minimumSpend: 1000 },
    SAVE20: { percentage: 20, minimumSpend: 5000 }
  };

  const discount = discountCodes[code];

  if (!discount || subtotal < discount.minimumSpend) {
    console.log("No discount applied.");
    return 0;
  }

  // Round currency to two decimal places.
  const amount = subtotal * discount.percentage / 100;
  console.log(`${code} applied: ${discount.percentage}% off.`);
  return Math.round(amount * 100) / 100;
}

const discount = calculateDiscount(subtotal, "SAVE10");
console.log(`Discount: ₹${discount.toFixed(2)}`);

console.log("\nSTEP 6: Calculate shipping and the final total");

function calculateShipping(subtotalAfterDiscount) {
  // Example business rule: free shipping on orders of ₹3,000 or more.
  return subtotalAfterDiscount >= 3000 ? 0 : 99;
}

const taxableAmount = Math.max(0, subtotal - discount);
const shipping = calculateShipping(taxableAmount);
const total = taxableAmount + shipping;

console.log(`Amount after discount: ₹${taxableAmount.toFixed(2)}`);
console.log(`Shipping: ₹${shipping.toFixed(2)}`);
console.log(`Order total: ₹${total.toFixed(2)}`);

console.log("\nSTEP 7: Generate an order summary");

function createOrderSummary() {
  const items = Array.from(cart.entries()).map(([productId, quantity]) => {
    const product = findProduct(productId);

    return {
      productId,
      name: product.name,
      quantity,
      unitPrice: product.price,
      lineTotal: product.price * quantity
    };
  });

  return {
    items,
    subtotal,
    discount,
    shipping,
    total,
    currency: "INR"
  };
}

const order = createOrderSummary();
console.log(JSON.stringify(order, null, 2));

console.log("\nSTEP 8: Prepare for a backend checkout");

function createCheckoutPayload() {
  // Send IDs and quantities, not trusted client-calculated prices.
  return {
    items: Array.from(cart.entries()).map(([productId, quantity]) => ({
      productId,
      quantity
    }))
  };
}

console.log("Checkout request payload:");
console.log(JSON.stringify(createCheckoutPayload(), null, 2));
console.log("The backend must validate prices, inventory, and payment status.");