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What is Nodemailer, and how does it let a Node.js application send emails via SMTP? What is the difference between the 'to', 'cc', and 'bcc' fields when sending an email programmatically, and why does 'bcc' matter for privacy? Why should sending an email typically happen in a background job rather than synchronously within an HTTP request handler? How would you send a push notification from a Node.js backend to a mobile app using Firebase Cloud Messaging (FCM)? What is the difference between transactional email and marketing email, and why do most Node.js applications use separate providers or sending domains for each? What is an unsubscribe mechanism, and what specific requirements does it need to meet for compliance with regulations like CAN-SPAM? What is the difference between sending SMS via a provider's REST API (like Twilio) versus using SMTP-based email in terms of a Node.js integration? How would you test that a Node.js application correctly sends emails without actually sending real emails during automated tests? What are SPF, DKIM, and DMARC, and why do they matter for a Node.js application's transactional emails actually reaching users' inboxes? How would you design a multi-channel notification system in Node.js that can send the same logical notification via email, SMS, or push depending on user preference?
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Node.js › Email & Notifications

What is email template rendering, and how might you use a templating engine to generate dynamic HTML emails in Node.js?

Intermediate

Answer

Rather than building HTML email bodies via manual string concatenation, a templating engine (like Handlebars, EJS, or a dedicated email-templating library like MJML) lets you define reusable email layouts with placeholders for dynamic content (a user's name, an order summary), rendered into final HTML at send time -- MJML specifically compiles a simplified markup into HTML/CSS that's been tested to render consistently across the notoriously inconsistent rendering engines used by different email clients.
const mjml2html = require('mjml');
const Handlebars = require('handlebars');

const template = Handlebars.compile(mjmlSource);
const filledMjml = template({ userName: user.name, orderTotal: order.total });
const { html } = mjml2html(filledMjml);

await transporter.sendMail({ to: user.email, subject: 'Order Confirmation', html });
Real-world example A team building order-confirmation emails uses MJML for the layout (guaranteeing consistent rendering across Outlook, Gmail, and Apple Mail) combined with Handlebars for injecting the specific order details, rather than hand-writing fragile, client-inconsistent raw HTML.

Common follow-ups: Why is writing HTML email templates significantly more constrained than writing regular web HTML/CSS?;How would you preview a rendered email template across different email clients before sending it to real users?

HTML & Web Servers;Testing with Jest Mocha & the Node Test Runner
All 15 Email & Notifications questions