added base examples and drills

kata/billing/original/invoiceService.js

@@ -0,0 +1,39 @@
+const fs = require("fs");
+
+function finalizeInvoice(filePath, invoiceId, customerType) {
+  const raw = fs.readFileSync(filePath, "utf8");
+  const state = JSON.parse(raw);
+  const invoice = state.invoices.find((item) => item.id === invoiceId);
+
+  if (!invoice) {
+    throw new Error("Invoice not found");
+  }
+
+  let subtotal = 0;
+
+  for (const line of invoice.lines) {
+    subtotal = subtotal + line.quantity * line.unitPrice;
+  }
+
+  let discount = 0;
+
+  if (customerType === "vip") {
+    discount = subtotal * 0.15;
+  }
+
+  if (subtotal > 200) {
+    discount = discount + 10;
+  }
+
+  const taxed = (subtotal - discount) * 1.1;
+  invoice.status = "finalized";
+  invoice.total = Math.round(taxed * 100) / 100;
+  invoice.finalizedAt = new Date().toISOString();
+
+  fs.writeFileSync(filePath, JSON.stringify(state, null, 2));
+  console.log("Invoice finalized", invoiceId);
+
+  return invoice;
+}
+
+module.exports = { finalizeInvoice };