Express v5: What You Need to Know About Express v5

Express 5 introduces some important changes that may affect the compatibility of applications built with Express 4. While the core API remains mostly unchanged, several deprecated methods and behaviors have been removed or updated, potentially causing issues during migration. Below is a concise breakdown of the changes, new behaviors, and migration tips to help you transition smoothly.

Installation Requirements

• Node.js version: 18 or higher
• Install Express 5:

npm install "express@>=5.0.0" --save

After installation, run your automated tests to identify any broken parts of your code and address the necessary updates based on the changes listed below.

Key Changes in Express 5

Removed Methods & Properties

Some previously supported methods and signatures have been removed entirely. Usage of these will now cause application crashes.

1. app.del()

• Use app.delete() instead for DELETE routes.
• Example:

app.delete('/users/:id', (req, res) => res.send('Deleted'));

2. app.param(fn)

• The general-purpose app.param(fn) is no longer available. Use app.param(name, fn) to modify route parameters.

3. Pluralized Methods

Methods renamed for better clarity:

• req.acceptsCharset() → req.acceptsCharsets()
• req.acceptsEncoding() → req.acceptsEncodings()
• req.acceptsLanguage() → req.acceptsLanguages()

4. req.param(name) Removed

Retrieve parameters directly from:

• req.params (route parameters)
• req.body (POST request body)
• req.query (query strings)

5. Changed Response Method Signatures

res.json(obj, status) → Use:

res.status(status).json(obj);

res.jsonp(obj, status) → Use:

res.status(status).jsonp(obj);

res.redirect(url, status) → Use:

res.redirect(status, url);

6. res.send(body, status) and res.send(status) Removed

• Use res.status() to chain responses:

res.status(404).send('Not Found');

• To send status codes alone:

res.sendStatus(404);

7. res.sendfile() → res.sendFile()

• Use the camel-cased version:

res.sendFile('/path/to/file.html');

8. Magic String 'back' Deprecated

res.redirect('back') → Use:

res.redirect(req.get('Referrer') || '/');

Changes to Route Matching Syntax

• Wildcard Syntax: Use /*splat instead of /*.
• No Optional Character Support: Replace /route? with /route{.:ext}.
• RegExp Patterns Removed: Define multiple routes explicitly:

app.get(['/discussion/:slug', '/page/:slug'], (req, res) => res.send('OK'));

Middleware and Async Error Handling

• Async Error Handling: If an async route handler throws an error or returns a rejected promise, it is automatically forwarded to the error handler:

app.get('/route', async (req, res, next) => {
  try {
    // Some async operation
  } catch (err) {
    next(err);  // Forward to error handler
  }
});

Updates to Express Core Functions

1. express.urlencoded()

• The extended option is now false by default.

2. req.body **Behavior Change**

• If the request body is not parsed, req.body returns undefined instead of {}.

3. req.host **Fix**

• Now retains the port number (if present) in the hostname.

4. req.query **Becomes Read-Only**

• The query object is now a getter and cannot be modified directly.

5. res.clearCookie() **Behavior**

• Ignores the maxAge and expires options.

6. res.status() **Validation**

• Now accepts only integer status codes between 100-999. Passing an invalid code will throw an error.

7. res.vary() **Strict Argument Check**

• An error is thrown if the field argument is missing.

New Improvements

1. res.render() Now Enforces Asynchronous Behavior

• This avoids bugs from synchronous template engines violating the async interface.

2. Brotli Compression Support

• Express 5 now supports Brotli encoding out of the box, improving compression for modern browsers.

3. app.router is Back

• The app.router object is available again but acts as a reference to the base Express router, unlike Express 3, where it needed to be explicitly loaded.

Migration Tips

1. Automate Testing: Run your test suite after upgrading to catch breaking changes quickly.
2. Update Deprecated Methods: Review your code for deprecated response methods like res.sendfile() and change them to the new supported versions.
3. Refactor Middleware for Async Errors: Ensure that your async handlers use try-catch blocks or return promises correctly.
4. Check for Query Parsing Changes: If your app relies heavily on query parameters, ensure the read-only behavior aligns with your code logic.

Conclusion

Migrating to Express 5 is straightforward if you follow the changes carefully. While most of the updates revolve around removing deprecated methods and improving error handling, they also provide enhanced reliability for modern web applications. Focus on refactoring outdated methods, testing thoroughly, and embracing async practices for better performance and smoother development.