Agent crash when defining an array of middleware to `router.use`

[taschmidt]

Description

When upgrading to newrelic @13.6.0, our code started breaking with the following exception:

/Users/schmidtt/w/video-api2/node_modules/newrelic/lib/subscribers/express/use.js:28
    if (fn.lazyrouter || fn.name === 'mounted_app') {
           ^
TypeError: Cannot read properties of undefined (reading 'lazyrouter')
    at ExpressUseSubscriber.handler (/Users/schmidtt/w/video-api2/node_modules/newrelic/lib/subscribers/express/use.js:28:12)
    at handler (/Users/schmidtt/w/video-api2/node_modules/newrelic/lib/subscribers/base.js:234:27)
    at node:diagnostics_channel:92:17
    at Channel.runStores (node:diagnostics_channel:177:12)
    at TracingChannel.traceSync (node:diagnostics_channel:333:18)
    at Function.use (/Users/schmidtt/w/video-api2/node_modules/express/lib/router/index.js:287:29)
    at Object.<anonymous> (/Users/schmidtt/w/video-api2/src/routes/api/index.ts:44:8)
    at Module.<anonymous> (node:internal/modules/cjs/loader:1706:14)
    at Module.wrappedCompile [as _compile] (/Users/schmidtt/w/video-api2/node_modules/@apm-js-collab/tracing-hooks/index.js:45:27)
    at Object..js (node:internal/modules/cjs/loader:1839:10)

I tracked the problem down to this change: #3424

It seems to be causing an incompatibility with some error handling middleware of ours so it's entirely possible we may need to change assumptions in there.

Expected Behavior

It shouldn't throw an exception.

Troubleshooting or NR Diag results

Steps to Reproduce

Pull down this repo and run npx tsx --env-file .env ./app.ts.

Your Environment

Node v22.21.1
MacOS 15.7.2

• ex: Browser name and version:
• ex: Node version:
• ex: Operating System and version:

Additional context

[Newrelic-Boomi]

https://new-relic.atlassian.net/browse/NR-488998

[jsumners-nr]

Thank you for the report. Your reproduction has defined an explicit "newrelic": "13.6.0" dependency. We have published several fixes due to previously unaccounted for edge cases. Please update to the latest 13.6.5 to see if this resolves your issue.

[bizob2828]

@taschmidt I can reproduce with your example linked. No need to check if it still exists on the latest. It appears the crash is because router.use is getting passed an array of middleware and the code you linked that is crashing isn't handling it. I'll add a test and make a fix, sorry for the trouble.s

[jsumners-nr]

I was just giving it a try myself and ran into this while trying to get plain JavaScript to work with:

❯ node --experimental-transform-types middleware.ts
(node:22525) ExperimentalWarning: Transform Types is an experimental feature and might change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
/private/tmp/18/newrelic-use-bug/middleware.ts:1
import { serializeError } from '@common.js/serialize-error';
         ^
SyntaxError: The requested module '@common.js/serialize-error' does not provide an export named 'serializeError'
    at ModuleJob._instantiate (node:internal/modules/esm/module_job:226:21)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async ModuleJob.run (node:internal/modules/esm/module_job:335:5)
    at async onImport.tracePromise.__proto__ (node:internal/modules/esm/loader:665:26)
    at async asyncRunEntryPointWithESMLoader (node:internal/modules/run_main:117:5)

[taschmidt]

@bizob2828 great news, thank you so much! Just wondering if there are downsides to this and if I should be refactoring our middleware at all?

@jsumners-nr strange, did you try running with tsx?

[bizob2828]

if there are downsides to this and if I should be refactoring our middleware at all?

That's more of a question for the express maintainers

[taschmidt]

if there are downsides to this and if I should be refactoring our middleware at all?

That's more of a question for the express maintainers

Gotcha. Sorry, I didn't dig into it too deeply, just trying to confirm if I was doing something dumb!