Error handling and debugging in EJS
Error handling and debugging in EJS (Embedded JavaScript) are crucial for ensuring that your templates render correctly and efficiently. EJS is a templating engine used to generate HTML pages dynamically by embedding JavaScript code. When working with EJS, you need to handle errors gracefully and debug issues that arise during template rendering.
Error Handling in EJS
1. Syntax Errors
EJS templates use JavaScript syntax within HTML. Common syntax errors include missing tags, incorrect use of delimiters, and unclosed brackets.
Example of Syntax Error:
<p>Hello, <%= user.name %></p>
<!-- Missing closing bracket or typo in EJS syntax -->
How to Identify:
- Syntax errors are often indicated by errors in the server console or error messages when the page is rendered.
- Check for unclosed EJS tags and incorrect JavaScript syntax within your EJS files.
Solution:
- Ensure all EJS tags are correctly opened and closed.
- Validate JavaScript code within EJS templates.
2. Handling Undefined Variables
Passing data to EJS templates from the server is common. If variables are missing or undefined, it can cause issues in rendering.
Example:
Template (views/profile.ejs
):
<p>Welcome, <%= user.name %>!</p>
Server Code (Express):
app.get('/profile', (req, res) => {
// Assuming user is undefined or null
res.render('profile', { user: undefined });
});
How to Identify:
- The rendered page might show
undefined
or break if the variable is not provided.
Solution:
- Use default values or conditional logic to handle undefined variables.
Template with Default Value:
<p>Welcome, <%= user && user.name ? user.name : 'Guest' %>!</p>
3. Handling Errors in Server-Side Code
Errors in server-side code that prepares data for EJS templates can affect rendering. Use try-catch blocks and error handling middleware to catch and handle these errors.
Example:
Server Code (Express):
app.get('/profile', async (req, res, next) => {
try {
const user = await User.findById(req.params.id);
res.render('profile', { user });
} catch (error) {
next(error); // Pass error to error handling middleware
}
});
Error Handling Middleware:
app.use((err, req, res, next) => {
console.error(err.stack);
res.status(500).send('Something broke!');
});
4. Debugging Tips
- Log Output: Use
console.log
statements in your server-side code to log data before rendering it. - Error Messages: Check server logs for detailed error messages and stack traces.
- Browser Console: Check the browser console for errors related to client-side rendering issues.
- Validate Templates: Use EJS validators or linters to ensure templates are correctly structured.
Debugging EJS Templates
1. Console Logging
You can include console.log
statements in your server-side code to inspect the data being passed to EJS templates.
Example:
Server Code (Express):
app.get('/profile', async (req, res) => {
try {
const user = await User.findById(req.params.id);
console.log('User data:', user); // Log user data to console
res.render('profile', { user });
} catch (error) {
console.error('Error fetching user:', error);
res.status(500).send('Error occurred');
}
});
2. Debugging Template Variables
Check the values of variables passed to the template. Ensure they are defined and have the expected data.
Example:
Template (views/profile.ejs
):
<p>Welcome, <%= user.name || 'Guest' %>!</p>
<% if (!user) { %>
<p>No user data available.</p>
<% } %>
3. Using EJS Debugging Tools
Some IDEs or text editors have built-in tools or plugins for debugging EJS templates. These can help identify syntax errors and other issues.
4. Structured Error Handling
Incorporate structured error handling in your application to catch and handle errors gracefully.
Example:
Error Handling Middleware:
app.use((err, req, res, next) => {
console.error('Error:', err.message);
res.status(500).render('error', { error: err.message });
});
Error Template (views/error.ejs
):
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Error</title>
</head>
<body>
<h1>An Error Occurred</h1>
<p><%= error %></p>
</body>
</html>
Summary
- Syntax Errors: Ensure correct EJS syntax and validate JavaScript code within templates.
- Undefined Variables: Handle missing or undefined variables with default values or conditional logic.
- Server-Side Errors: Use try-catch blocks and error handling middleware to manage errors in server-side code.
- Debugging Tips: Use console logging, check browser console, and use EJS validators for debugging.
- Structured Error Handling: Implement error handling middleware to catch and render errors gracefully.
By effectively handling errors and debugging EJS templates, you can ensure that your application runs smoothly and provides a better user experience.