Express 5.1: req.body is `undefined` when using express.json() for requests that do not have a request body

[booherbg]

Simply put, with express 5.1 req.body is undefined when not explicitly including a request body. This is unexpected! In express 4.x, req.body is guaranteed to be at least {} even when the request body is missing.

Express 5.1 Bug Report: express.json() Middleware Body Initialization

Environment information

Version:

"dependencies": {
  "express": "5.1.0"  // Broken behavior
  "express": "4.21.2" // Working behavior
}

Platform:

$ uname -a
 24.2.0 Darwin Kernel Version 24.2.0: Fri Dec  6 18:40:14 PST 2024; root:xnu11215.61.5~2/RELEASE_ARM64_T8103 arm64

Node.js version:

$ node -v
v22.9.0

What steps will reproduce the bug?

1. Create minimal Express server:

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

app.use(express.json());

app.post('/test', (req, res) => {
  console.log('Body:', req.body);
  res.json({ receivedBody: req.body });
});

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

2. Send POST request with no body:

$ npm install express@5
$ curl -X POST http://localhost:3000/test
Server Output: `Body: undefined`
$ curl -X POST http://localhost:3000/test -H "Content-Type: application/json"
Server Output: `Body: undefined`

$ npm install express@4
$ curl -X POST http://localhost:3000/test
Server Output: `Body: {}`
$ curl -X POST http://localhost:3000/test -H "Content-Type: application/json"
Server Output: `Body: {}`

3. Observe behavior:

• Express 4.x: req.body is {}
• Express 5.x: req.body is undefined

Expected Behavior

When a POST request is made with no body (with or without Content-Type: application/json), the express.json() middleware should initialize req.body to an empty object {}, as it did in Express 4.x.

Actual Behavior

In Express 5.x, req.body is undefined when no body is sent, even when the request includes Content-Type: application/json.

This change breaks backward compatibility and affects common REST API patterns where routes expect req.body to be an object, even if empty.

Example output:

$ npm install express@5
$ curl -X POST http://localhost:3000/test
# Server Output: `Body: undefined`
$ npm install express@4
# Server Output: `Body: {}`

[dpopp07]

This was actually an intentional change in v5 and is documented in the migration guide.

This enables an app to disambiguate an empty body and a non-existent body.

[booherbg]

Ahh. Interesting, that's definitely a breaking change. I did not uncover that information while googling so hopefully folks can find this thread. I'm surprised this isn't an "opt-in" feature (like app.use(express.json({ allowEmptyBody: true})))

Thank you for the quick reply!

[krzysdz]

I did not uncover that information while googling so hopefully folks can find this thread.

There is already #6005

[dpopp07]

Ahh. Interesting, that's definitely a breaking change.

Indeed, hence the inclusion in the v5 major release 🙂 We've tried to document all breaking changes in that migration guide - I'd glance over that if you want to try and avoid any more surprises!

[LorenzoLeonardini]

I'd like to point out that the API reference says

A new body object containing the parsed data is populated on the request object after the middleware (i.e. req.body), or an empty object ({}) if there was no body to parse, the Content-Type was not matched, or an error occurred.

[wesleytodd]

@LorenzoLeonardini we would love a PR to fix that. We tried to get all the related docs updated but must have missed that one.

[wesleytodd]

Also since I don't see it mentioned here: it is called out in the breaking changes section of our release post and in all the relevant changelogs.