Template Variables Tutorial | Express.js

Introduction

Template variables are placeholders that allow you to pass data from your Express.js application to your views/templates. They enable you to dynamically populate your views with dynamic content. This tutorial will guide you through the process of using template variables in Express.js, providing code examples and step-by-step explanations.

Example Code

const express = require('express');
const app = express();

app.set('view engine', 'ejs');

app.get('/', (req, res) => {
const data = {
title: 'My Website',
message: 'Welcome to my website!',
users: ['John', 'Jane', 'Bob']
};
res.render('index', data);
});

app.listen(3000, () => {
console.log('Server listening on port 3000');
});

Steps to Use Template Variables

1. Set the Template Engine: Install a template engine like EJS, Pug, or Handlebars using npm or yarn. Set the template engine in your Express.js application using the app.set() method with the 'view engine' setting. For example, app.set('view engine', 'ejs') sets EJS as the template engine.
2. Pass Data to Views: In your route handler, define an object containing the data you want to pass to the view. Each key-value pair represents a template variable name and its corresponding value. For example, const data = { title: 'My Website', message: 'Welcome!' };
3. Render Views with Data: Use the res.render() method to render the view with the data. Pass the view name as the first parameter and the data object as the second parameter. The template engine will merge the data with the view and generate the final HTML output. For example, res.render('index', data);
4. Access Template Variables in Views: In your view/template files, you can access the template variables using the syntax defined by the template engine. For example, in EJS, you can use <h1><%= title %></h1> to display the value of the "title" variable.

Common Mistakes

• Not setting the template engine correctly using app.set(), resulting in failure to recognize and render the template variables.
• Forgetting to pass the data object when calling res.render(), resulting in missing template variables in the view.
• Using incorrect syntax or misspelling the template variable names in the view files, causing rendering errors or displaying incorrect data.
• Attempting to access template variables that are not defined or passed to the view, leading to undefined values or errors.

Frequently Asked Questions (FAQs)

1. Can I pass complex data structures like arrays or objects as template variables?

   Yes, you can pass complex data structures like arrays or objects as template variables. Ensure that the data you pass is compatible with the template engine's syntax and that you properly handle the data in your view/template files.

2. Can I use template variables in conditional statements or loops in my view/template files?

   Yes, template engines provide syntax and constructs to include conditional statements and loops in your view/template files. This allows you to conditionally render content or iterate over data using template variables. Refer to the documentation of the specific template engine you are using for details on how to use conditional statements and loops.

3. What happens if I pass a template variable that is not used in the view/template?

   If you pass a template variable that is not used in the view/template, it will be ignored by the template engine. The variable will not have any effect on the rendering process or output.

4. Can I modify the template variables within the view/template files?

   No, the template variables are typically read-only and should not be modified within the view/template files. Their purpose is to display dynamic data as provided by the server.

5. How can I include raw HTML or special characters in the template variables?

   To include raw HTML or special characters in the template variables without escaping, you can use the appropriate syntax provided by the template engine. Refer to the documentation of the specific template engine you are using for details on how to include raw HTML or special characters.

Summary

Template variables are a powerful feature in Express.js that allow you to pass data from your server to your views/templates. By setting the template engine, passing data to views, rendering views with data, and accessing template variables in your view/template files, you can dynamically populate your views with dynamic content. Avoid common mistakes such as misconfigurations, missing data, or incorrect syntax when working with template variables. With template variables, you can create dynamic and personalized web applications efficiently.