Express.js Template Engines: A Beginner's Guide to Rendering Dynamic Content

When you first start building web applications with Node.js and Express.js, you quickly move beyond sending simple text responses. The real power lies in serving dynamic, data-driven web pages. This is where Express template engines become your essential toolkit. They allow you to seamlessly blend your server-side JavaScript logic with HTML, creating personalized, interactive user experiences. For anyone aiming for certification or a career in full-stack development, mastering template engines is a non-negotiable skill. This guide will demystify popular engines like EJS and Pug, explain core template syntax, and show you how to implement dynamic rendering like a pro.

Why Template Engines Are Crucial for Modern Web Development

Imagine building an e-commerce site. Every product page has the same structure (header, image gallery, description, reviews), but the content is unique for each item. Writing a separate HTML file for thousands of products is impossible. A template engine solves this by letting you create one template file. Your Express app then injects different product data into this template, generating a unique page for each product on the fly. This approach, known as server-side rendering (SSR), is fundamental for SEO-friendly, performant web applications and is a core concept tested in many full-stack certifications.

Choosing Your Engine: EJS vs. Pug (Jade)

Express doesn't force a particular template engine on you. This flexibility is great but can be confusing for beginners. Two of the most popular and beginner-friendly choices are EJS and Pug. Your choice often comes down to a preference in syntax and philosophy.

EJS (Embedded JavaScript)

EJS stands for "Embedded JavaScript." Its biggest selling point is that it looks and feels like regular HTML. You write standard HTML tags and embed JavaScript logic within special <% %> delimiters. If you're already comfortable with HTML, EJS has a very gentle learning curve.

Syntax Style: HTML-like, minimal abstraction.

Best For: Beginners, teams who want templates that are immediately recognizable as HTML, and projects where front-end designers need to work with the template files.

Pug (Formerly Jade)

Pug takes a radically different approach. It uses a clean, indentation-based syntax that eliminates the need for closing tags and angle brackets. It's highly concise and can significantly reduce the amount of code you write. However, its unique syntax requires a dedicated learning period.

Syntax Style: Indentation-based, whitespace-sensitive, highly abstracted.

Best For: Developers who value conciseness, projects where backend developers are primarily responsible for templates, and those who enjoy a clean, Python-like syntax.

Understanding the trade-offs between these engines is a practical skill often overlooked in theory-heavy courses. In our hands-on Full Stack Development course, we build projects with both, so you can make an informed choice based on real experience, not just documentation.

Core Template Syntax: The Building Blocks of Dynamic Rendering

Regardless of the engine you choose, all template languages provide core constructs for dynamic content. Let's break them down with parallel examples in EJS and Pug.

1. Variable Interpolation (Outputting Data)

This is the most basic operation: inserting a value from your server into the HTML.

Express Route (app.js):

app.get('/user', (req, res) => {
  res.render('user-profile', {
    userName: 'Alex Johnson',
    userRole: 'Admin'
  });
});

EJS Template (user-profile.ejs):

<h1>Welcome, <%= userName %>!</h1>
<p>Your role is: <%= userRole %></p>

Pug Template (user-profile.pug):

h1 Welcome, #{userName}!
p Your role is: #{userRole}

2. Conditionals (If/Else Logic)

Control what gets rendered based on your data.

EJS Example:

<% if (user.isLoggedIn) { %>
  <a href="/logout">Logout</a>
<% } else { %>
  <a href="/login">Login</a>
<% } %>

Pug Example:

if user.isLoggedIn
  a(href="/logout") Logout
else
  a(href="/login") Login

3. Loops (Iterating Over Arrays)

Generate lists, tables, or galleries from arrays of data.

Express Route:

res.render('product-list', {
  products: [
    { name: 'Laptop', price: 999 },
    { name: 'Mouse', price: 25 },
    { name: 'Keyboard', price: 75 }
  ]
});

EJS Example (Creating a list):

<ul>
  <% products.forEach(function(product) { %>
    <li><%= product.name %> - $<%= product.price %></li>
  <% }); %>
</ul>

Pug Example (Creating a list):

ul
  each product in products
    li #{product.name} - $#{product.price}

Structuring and Organizing Your Templates

As your app grows, you can't repeat the same header and footer HTML in every file. Template engines provide mechanisms for layout and partials (components).

• Layouts/Base Templates: A master template that defines the overall page structure (DOCTYPE, head, header, footer). Individual page templates then "extend" this layout and inject content into specific blocks.
• Partials/Includes: Reusable template snippets like a navigation bar, comment section, or card component. You can include these in any other template.

Example: Using a Partial in EJS

<!-- views/partials/navbar.ejs -->
<nav><a href="/">Home</a> | <a href="/about">About</a></nav>

<!-- views/home.ejs -->
<body>
  <%- include('partials/navbar') %>
  <h1>Home Page</h1>
</body>

Learning to structure templates efficiently is a hallmark of a professional developer. It’s a practical skill we emphasize heavily in our project-based curriculum at LeadWithSkills, moving beyond simple examples to architecting real applications.

Setting Up and Testing a Template Engine in Express.js

Let's walk through a manual setup for EJS, which is excellent practice for certification exams that test foundational knowledge.

1. Install EJS: Run npm install ejs in your project directory.
2. Configure Express: In your main app file (e.g., app.js or server.js):

   const express = require('express');
   const app = express();
   
   // Set the view engine to EJS
   app.set('view engine', 'ejs');
   // Tell Express where your template files are located (default is './views')
   app.set('views', './views');

3. Create a Views Directory: Create a folder named views in your project root.
4. Create a Template: Inside views, create index.ejs with basic HTML and an EJS variable.
5. Create a Route to Render:

   app.get('/', (req, res) => {
     res.render('index', { pageTitle: 'My Homepage', currentYear: new Date().getFullYear() });
   });

6. Test It: Run your server with node app.js and visit http://localhost:3000. You should see your dynamic page. Manually change the data passed in the res.render() function and refresh to see the content update—this is dynamic rendering in action.

Beyond the Basics: Preparing for Real-World Projects

While understanding syntax is the first step, building professional applications requires more:

• Passing Complex Data: Learning to pass and manipulate objects, arrays of objects, and nested data structures to your templates.
• Template Context & Locals: Using middleware to pass global data (like a user session) to all templates automatically.
• Error Handling in Templates: Safely handling missing data to prevent template crashes.
• Integration with Frontend Frameworks: Understanding when to use server-side rendering (with EJS/Pug) vs. client-side rendering (with frameworks like Angular or React). This is a critical architectural decision.

For example, while Express with EJS handles the server-rendered view layer beautifully, you might use Angular for a complex, single-page application (SPA) dashboard within the same full-stack project. Knowing how these pieces fit together is what makes you a valuable, versatile developer.

Common Pitfalls and Best Practices

As you practice, keep these points in mind:

• Don't Put Heavy Logic in Templates: Templates are for presentation. Complex business logic, calculations, or database queries should happen in your route controllers before you call res.render().
• Escape Output by Default: Use <%= %> (EJS) or #{} (Pug) as they escape HTML characters, preventing XSS attacks. Only use the unescaped versions (<%- %> in EJS) when you intentionally need to render HTML content.
• Keep Templates Focused: If a template file becomes too long and complex, break it down using partials and layouts.

Conclusion: Your Path to Mastering Dynamic Content

Express.js template engines are the bridge between your application's logic and its user interface. Mastering EJS, Pug, and the principles of dynamic rendering empowers you to build data-driven, professional web applications. Start by practicing the core template syntax—interpolation, conditionals, and loops. Then, progressively tackle layouts and partials to write clean, maintainable code.

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