feat: add SMTP mailer for price-change notifications

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/mailer.js

@@ -0,0 +1,30 @@
+const nodemailer = require('nodemailer');
+
+function createTransport(env = process.env) {
+  return nodemailer.createTransport({
+    host: env.SMTP_HOST,
+    port: Number(env.SMTP_PORT || 587),
+    secure: Number(env.SMTP_PORT) === 465,
+    auth: env.SMTP_USER ? { user: env.SMTP_USER, pass: env.SMTP_PASS } : undefined,
+  });
+}
+
+function formatPrice(value) {
+  return value.toFixed(2).replace('.', ',');
+}
+
+async function sendPriceChangeEmail(transport, { from, to, name, url, oldPrice, newPrice }) {
+  await transport.sendMail({
+    from,
+    to,
+    subject: `Prijs gewijzigd: ${name}`,
+    text: [
+      `De prijs van "${name}" is gewijzigd.`,
+      `Oude prijs: €${formatPrice(oldPrice)}`,
+      `Nieuwe prijs: €${formatPrice(newPrice)}`,
+      `Link: ${url}`,
+    ].join('\n'),
+  });
+}
+
+module.exports = { createTransport, sendPriceChangeEmail, formatPrice };

test/mailer.test.js

@@ -0,0 +1,43 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const { sendPriceChangeEmail, formatPrice, createTransport } = require('../src/mailer');
+
+test('formatPrice formats with comma decimal separator', () => {
+  assert.equal(formatPrice(19.9), '19,90');
+  assert.equal(formatPrice(24.5), '24,50');
+});
+
+test('sendPriceChangeEmail sends mail with expected fields', async () => {
+  const calls = [];
+  const fakeTransport = { sendMail: async (opts) => { calls.push(opts); } };
+  await sendPriceChangeEmail(fakeTransport, {
+    from: 'korting@sjoerd.app',
+    to: 'me@example.com',
+    name: 'Voorbeeld product',
+    url: 'https://shop.example/product',
+    oldPrice: 19.99,
+    newPrice: 17.5,
+  });
+  assert.equal(calls.length, 1);
+  assert.equal(calls[0].from, 'korting@sjoerd.app');
+  assert.equal(calls[0].to, 'me@example.com');
+  assert.equal(calls[0].subject, 'Prijs gewijzigd: Voorbeeld product');
+  assert.match(calls[0].text, /19,99/);
+  assert.match(calls[0].text, /17,50/);
+  assert.match(calls[0].text, /https:\/\/shop\.example\/product/);
+});
+
+test('sendPriceChangeEmail propagates transport errors', async () => {
+  const fakeTransport = { sendMail: async () => { throw new Error('smtp down'); } };
+  await assert.rejects(
+    () => sendPriceChangeEmail(fakeTransport, {
+      from: 'a@b.com', to: 'c@d.com', name: 'X', url: 'https://x', oldPrice: 1, newPrice: 2,
+    }),
+    /smtp down/
+  );
+});
+
+test('createTransport builds a transport from env vars', () => {
+  const transport = createTransport({ SMTP_HOST: 'mail.sjoerd.app', SMTP_PORT: '587', SMTP_USER: 'u', SMTP_PASS: 'p' });
+  assert.equal(typeof transport.sendMail, 'function');
+});