When working with EJS (Embedded JavaScript), developers might encounter various common errors related to syntax, data handling, and configuration. Understanding these errors and their causes can help in troubleshooting and fixing issues effectively.

Common EJS Errors

1. Syntax Errors

Description: Syntax errors occur when there is a mistake in the EJS template code. This can include unclosed tags, incorrect delimiters, or improper JavaScript syntax.

Examples:

• Unclosed EJS Tag:

  <p>Hello, <%= user.name %> <!-- Missing closing tag -->
  

• Incorrect Delimiter Usage:

  <% if (user) { %>
    <p>Welcome, <%= user.name %></p>
  <% else %>
    <p>Please log in.</p>
  <% } %> <!-- Using <% } %> instead of <% } %> -->
  

How to Fix:

• Ensure that all EJS tags (<%, <%=, <%-, <%#, etc.) are properly opened and closed.
• Verify that all JavaScript code within EJS tags is correct and follows proper syntax rules.

2. Undefined Variables

Description: Passing undefined or null variables to EJS templates can result in rendering issues, such as displaying "undefined" or breaking the template.

Examples:

• Using Undefined Variable:

  <p>Hello, <%= user.name %>!</p> <!-- If user is undefined, it will cause issues -->
  

How to Fix:

• Ensure that all variables passed to EJS templates from the server are defined.
• Use default values or conditional logic in templates to handle cases where data might be missing.

Example Fix:

<p>Hello, <%= user && user.name ? user.name : 'Guest' %>!</p>

3. File Not Found

Description: If EJS cannot find the specified template file, it will throw an error indicating that the file does not exist.

Examples:

• Incorrect Path:

  res.render('nonexistentTemplate'); // Template not found
  

How to Fix:

• Verify that the template file exists in the correct directory.
• Ensure that the path specified in the res.render method matches the actual file location.

Example Fix:

app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'ejs');

4. Incorrect Configuration

Description: Incorrect configuration of the EJS view engine in your Express application can lead to errors when rendering templates.

Examples:

• Not Setting the View Engine:

  app.set('view engine', 'ejs'); // Must be set to use EJS as the template engine
  

How to Fix:

• Ensure that you have correctly configured EJS in your Express app using app.set('view engine', 'ejs').

5. JavaScript Errors within EJS

Description: Errors in JavaScript code within EJS templates can cause rendering issues or unexpected behavior.

Examples:

• Incorrect JavaScript Logic:

  <% if (user) { %>
    <p>Welcome, <%= user.name.toUpperCase() %>!</p> <!-- Throws an error if user.name is not a string -->
  <% } %>
  

How to Fix:

• Validate that all JavaScript logic within EJS templates is correct and handles edge cases.
• Use debugging tools and console logging to inspect the values and logic within your EJS templates.

Example Fix:

<% if (user && typeof user.name === 'string') { %>
  <p>Welcome, <%= user.name.toUpperCase() %>!</p>
<% } %>

6. Data Type Mismatches

Description: Passing data of an unexpected type to EJS templates can cause issues with rendering.

Examples:

• Passing an Object Instead of a String:

  <p>Welcome, <%= user.name %>!</p> <!-- If user.name is an object, it will cause issues -->
  

How to Fix:

• Ensure that data types match the expected format in your templates.
• Use type-checking and validation on the server side before passing data to templates.

Example Fix:

app.get('/profile', (req, res) => {
    const user = { name: 'John Doe' }; // Ensure data is in the expected format
    res.render('profile', { user });
});

7. Misconfigured Middleware

Description: Middleware issues, such as not using express.json() or express.urlencoded(), can affect data handling for forms.

Examples:

• Missing Middleware:

  app.use(express.json()); // Required for parsing JSON bodies
  

How to Fix:

• Ensure that all necessary middleware for handling requests and data parsing is configured in your Express app.

Example Fix:

app.use(express.urlencoded({ extended: true }));
app.use(express.json());

Summary

• Syntax Errors: Ensure correct use of EJS tags and JavaScript syntax within templates.
• Undefined Variables: Handle undefined variables by using default values or conditional logic.
• File Not Found: Verify the existence and correct path of template files.
• Incorrect Configuration: Properly configure EJS as the view engine in your Express app.
• JavaScript Errors: Validate JavaScript logic within EJS templates and handle edge cases.
• Data Type Mismatches: Ensure data types match the expected format in your templates.
• Misconfigured Middleware: Configure necessary middleware for handling data parsing and requests.

By addressing these common errors and implementing best practices, you can effectively manage EJS templates and ensure smooth rendering and functionality in your application.