Uncaught exception causes Express to set to undefined basic request parameters

[NicoAdrian]

Consider this code:

const express = require('express')
const app = express()
const port = 3000

app.get('/foo/:bar', (req, res) => {
  res.on('finish', () => console.log('onFinish: ' + req.params.bar));
  // uncaught exception
  'nothing' in undefined;
})

app.listen(port, () => {
  console.log(`Example app listening on port ${port}`)
})

curl http://localhost:3000/foo/blablabla will cause Express to:

1. Send HTTP 500 (expected)
2. Crash (not expected)

==> TypeError: Cannot read properties of undefined (reading 'bar')

After the uncaught exception, req.params is undefined (among other req parameters, not listed here). Thus, trying to access bar from undefined is also an exception, causing the server to crash and terminate.

Looks like Express "clears" some variables in the req object.

[danizavtz]

Looks like a problem with the event you are handling the event finish. if we check the documentation this event:

Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the operating system for transmission over the network.

so I'm guessing that in this logical scope it is no longer possible to access the object property you want. And that is why you receive an undefined.

If you change finish to end everything work as you expected.

[IamLizu]

Hi @NicoAdrian 👋

I think the uncaught exception in Node causes a crash unless properly handled. And trying to access the req.params after that uncaught exception will be unpredictable because the request life cycle is not the same.

I would add a global error handler,

app.use((err, req, res, next) => {
    console.log("error handler: " + err.message);
    res.status(500).send("Something broke!");
});

[NicoAdrian]

Looks like a problem with the event you are handling the event finish. if we check the documentation this event:

Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the operating system for transmission over the network.

so I'm guessing that in this logical scope it is no longer possible to access the object property you want. And that is why you receive an undefined.

If you change finish to end everything work as you expected.

The req express (or is it ?) object is still accessible, with many properties. But the property params (also the baseUrl property for instance).

The end and finish events serve different purposes:

• end is related to the end of reading data from the incoming request
• finish (what I want) is related to the completion of writing data to the outgoing response (writable stream)

Scope shouldn't be an issue: I still have the req.params in scope. I go back to my first question: Why is it cleared by express ? (or NodeJS ?)

[wesleytodd]

Why is it cleared by express ?

It is set and cleared by express. The pattern you are using cannot work because requests can cascade into other matched routes, which would override the params. There are some related open issues about storing matched routes in the req, but none of those have landed AFAIK. If you want this you need to save off a copy in your app code.

[wesleytodd]

Here is one of those PRs: pillarjs/router#86

[IamLizu]

It is set and cleared by express. The pattern you are using cannot work because requests can cascade into other matched routes, which would override the params. There are some related open issues about storing matched routes in the req, but none of those have landed AFAIK. If you want this you need to save off a copy in your app code.

Oh. I thought because of un-caught exception, the 'req-param' becomes inaccesibles because of request lifecycle.

Thanks for clearing it up.

[wesleytodd]

No problem! Yeah the router clears the things it sets after it passes out of context so that any chained routes don't accidentally leak things. https://github.com/pillarjs/router/blob/master/index.js#L175

[NicoAdrian]

No problem! Yeah the router clears the things it sets after it passes out of context so that any chained routes don't accidentally leak things. https://github.com/pillarjs/router/blob/master/index.js#L175

I'm not using PillarJS at all

[NicoAdrian]

okay, waiting for your explanations on this matter :)

[IamLizu]

okay, waiting for your explanations on this matter :)

Apparently I was wrong and @wesleytodd has cleared it up.

I am going to mark this answer of mine off topic to avoid confusion.