Moving to Express 5

Overview

Express 5.0 is still in the alpha release stage, but here is a preview of the changes that will be in the release and how to migrate your Express 4 app to Express 5.

Express 5 is not very different from Express 4: The changes to the API are not as significant as from 3.0 to 4.0. Although the basic API remains the same, there are still breaking changes; in other words an existing Express 4 program might not work if you update it to use Express 5.

To install the latest alpha and to preview Express 5, enter the following command in your application root directory:

$ npm install express@5.0.0-alpha.2 --save

You can then run your automated tests to see what fails, and fix problems according to the updates listed below. After addressing test failures, run your app to see what errors occur. You’ll find out right away if the app uses any methods or properties that are not supported.

Changes in Express 5

Here is the list of changes (as of the alpha 2 release ) that will affect you as a user of Express. See the pull request for a list of all the planned features.

Removed methods and properties

Changed

Improvements

Removed methods and properties

If you use any of these methods or properties in your app, it will crash. So, you’ll need to change your app after you update to version 5.

app.del()

Express 5 no longer supports the app.del() function. If you use this function an error is thrown. For registering HTTP DELETE routes, use the app.delete() function instead.

Initially del was used instead of delete, because delete is a reserved keyword in JavaScript. However, as of ECMAScript 6, delete and other reserved keywords can legally be used as property names.

app.param(fn)

The app.param(fn) signature was used for modifying the behavior of the app.param(name, fn) function. It has been deprecated since v4.11.0, and Express 5 no longer supports it at all.

Pluralized method names

The following method names have been pluralized. In Express 4, using the old methods resulted in a deprecation warning. Express 5 no longer supports them at all:

req.acceptsCharset() is replaced by req.acceptsCharsets().

req.acceptsEncoding() is replaced by req.acceptsEncodings().

req.acceptsLanguage() is replaced by req.acceptsLanguages().

Leading colon (:) in the name for app.param(name, fn)

A leading colon character (:) in the name for the app.param(name, fn) function is a remnant of Express 3, and for the sake of backwards compatibility, Express 4 supported it with a deprecation notice. Express 5 will silently ignore it and use the name parameter without prefixing it with a colon.

This should not affect your code if you follow the Express 4 documentation of app.param, as it makes no mention of the leading colon.

req.param(name)

This potentially confusing and dangerous method of retrieving form data has been removed. You will now need to specifically look for the submitted parameter name in the req.params, req.body, or req.query object.

res.json(obj, status)

Express 5 no longer supports the signature res.json(obj, status). Instead, set the status and then chain it to the res.json() method like this: res.status(status).json(obj).

res.jsonp(obj, status)

Express 5 no longer supports the signature res.jsonp(obj, status). Instead, set the status and then chain it to the res.jsonp() method like this: res.status(status).jsonp(obj).

res.send(body, status)

Express 5 no longer supports the signature res.send(obj, status). Instead, set the status and then chain it to the res.send() method like this: res.status(status).send(obj).

res.send(status)

Express 5 no longer supports the signature res.send(status), where status is a number. Instead, use the res.sendStatus(statusCode) function, which sets the HTTP response header status code and sends the text version of the code: “Not Found”, “Internal Server Error”, and so on. If you need to send a number by using the res.send() function, quote the number to convert it to a string, so that Express does not interpret it as an attempt to use the unsupported old signature.

res.sendfile()

The res.sendfile() function has been replaced by a camel-cased version res.sendFile() in Express 5.

Changed

app.router

The app.router object, which was removed in Express 4, has made a comeback in Express 5. In the new version, this object is a just a reference to the base Express router, unlike in Express 3, where an app had to explicitly load it.

req.host

In Express 4, the req.host function incorrectly stripped off the port number if it was present. In Express 5 the port number is maintained.

req.query

In Express 4.7 and Express 5 onwards, the query parser option can accept false to disable query string parsing when you want to use your own function for query string parsing logic.

Improvements

res.render()

This method now enforces asynchronous behavior for all view engines, avoiding bugs caused by view engines that had a synchronous implementation and that violated the recommended interface.