Cannot read property 'headersSent' of undefined

[marcus-sa]

I keep getting this error and from the log I can see that Express is using this module, and I have no clue on how to solve it.

/home/sentinel/Gitlab/stormcms-engine-v2/node_modules/finalhandler/index.js:239
[1]   return typeof res.headersSent !== 'boolean'
[1]                     ^
[1] 
[1] TypeError: Cannot read property 'headersSent' of undefined
[1]     at headersSent (/home/sentinel/Gitlab/stormcms-engine-v2/node_modules/finalhandler/index.js:239:21)
[1]     at /home/sentinel/Gitlab/stormcms-engine-v2/node_modules/finalhandler/index.js:92:17
[1]     at /home/sentinel/Gitlab/stormcms-engine-v2/node_modules/express/lib/router/index.js:635:15
[1]     at next (/home/sentinel/Gitlab/stormcms-engine-v2/node_modules/express/lib/router/index.js:210:14)
[1]     at Function.handle (/home/sentinel/Gitlab/stormcms-engine-v2/node_modules/express/lib/router/index.js:174:3)
[1]     at Function.handle (/home/sentinel/Gitlab/stormcms-engine-v2/node_modules/express/lib/application.js:174:10)
[1]     at app (/home/sentinel/Gitlab/stormcms-engine-v2/node_modules/express/lib/express.js:39:9)
[1]     at Object.<anonymous> (/home/sentinel/Gitlab/stormcms-engine-v2/src/server.js:104:1)
[1]     at Module._compile (module.js:635:30)
[1]     at loader (/home/sentinel/Gitlab/stormcms-engine-v2/node_modules/babel-register/lib/node.js:144:5)
[1]     at Object.require.extensions.(anonymous function) [as .js] (/home/sentinel/Gitlab/stormcms-engine-v2/node_modules/babel-register/lib/node.js:154:7)
[1]     at Module.load (module.js:554:32)
[1]     at tryModuleLoad (module.js:497:12)
[1]     at Module._load (module.js:489:3)
[1]     at Function.module._load (/home/sentinel/Gitlab/stormcms-engine-v2/node_modules/piping/lib/piping.js:218:16)
[1]     at Module.require (module.js:579:17)

[stratosgear]

Same here, while using latest version (v3) of feathersJs :scratch-head:

[dougwilson]

Thanks for the report, sorry I didn't see it earlier. Can one of you provide the following information?

1. Version of Node.js
2. Version of this module
3. Complete code to run to reproduce the issue
4. Instructions on how to replicate

Thanks!

[stratosgear]

node --version
v9.5.0

finalhandler
1.1.0

You could try to pull the repo I'm trying to fix:

https://github.com/stratosgear/feathers-vuejs

If you clone, then npm install and npm start you should get:

> plumage@0.2.0 start /mnt/data/projects/feathers-vuejs
> babel-node src/

/mnt/data/projects/feathers-vuejs/node_modules/finalhandler/index.js:239
  return typeof res.headersSent !== 'boolean'
                    ^

TypeError: Cannot read property 'headersSent' of undefined
    at headersSent (/mnt/data/projects/feathers-vuejs/node_modules/finalhandler/index.js:239:21)
    at /mnt/data/projects/feathers-vuejs/node_modules/finalhandler/index.js:92:17
    at Function.handle (/mnt/data/projects/feathers-vuejs/node_modules/express/lib/application.js:170:5)
    at Function.app (/mnt/data/projects/feathers-vuejs/node_modules/express/lib/express.js:39:9)
    at Function.configure (/mnt/data/projects/feathers-vuejs/node_modules/@feathersjs/feathers/lib/application.js:59:8)
    at Object.<anonymous> (/mnt/data/projects/feathers-vuejs/src/app.js:25:4)
    at Module._compile (module.js:660:30)
    at loader (/mnt/data/projects/feathers-vuejs/node_modules/babel-register/lib/node.js:144:5)
    at Object.require.extensions.(anonymous function) [as .js] (/mnt/data/projects/feathers-vuejs/node_modules/babel-register/lib/node.js:154:7)
    at Module.load (module.js:573:32)
    at tryModuleLoad (module.js:513:12)
    at Function.Module._load (module.js:505:3)
    at Module.require (module.js:604:17)
    at require (internal/module.js:11:18)
    at Object.<anonymous> (/mnt/data/projects/feathers-vuejs/src/index.js:3:13)
    at Module._compile (module.js:660:30)
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! plumage@0.2.0 start: `babel-node src/`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the plumage@0.2.0 start script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/stratos/.npm/_logs/2018-02-19T21_38_52_746Z-debug.log

[dougwilson]

Thanks @stratosgear I was able to reproduce the issue with your example. There is a lot going on here, though, so I won't be able to look at it right now since I was just about to leave. Is there any way you can trim down the reproduction to only using this module? The stack trace doesn't indicated an error in this module -- it is indicating that the function this module is exporting is being called without the required res argument:

node -pe 'require("finalhandler")()()'
node_modules\finalhandler\index.js:239
  return typeof res.headersSent !== 'boolean'
                   ^

TypeError: Cannot read property 'headersSent' of undefined
    at headersSent (node_modules\finalhandler\index.js:239:20)
    at node_modules\finalhandler\index.js:92:17
    at [eval]:1:26
    at ContextifyScript.Script.runInThisContext (vm.js:25:33)
    at Object.runInThisContext (vm.js:97:38)
    at Object.<anonymous> ([eval]-wrapper:6:22)
    at Module._compile (module.js:570:32)
    at evalScript (bootstrap_node.js:353:27)
    at run (bootstrap_node.js:122:11)
    at run (bootstrap_node.js:389:7)

If you provide the required res argument to this module's export, does that resolve the issue?

[stratosgear]

from what i googled around (nodejs/node#15226) you might get this if the stream is already closed...? so res is an undefined?

I'm over my head trying to port that old repo to the latest version of FeatherJs and I do not feel confident I would be able to provide a minimal working example of this, I'm afraid... :(

[dougwilson]

But the error happens on nom start without making a request yet, so I don't think it's related to actually making a request, right? The finalhandler module takes two required arguments to construct -- req and res. What request is happening during npm start that would provide with a req and res to even construct a new finalhandler instance?

[dougwilson]

I went through src/app.js and commented out each line one-by-one trying to see when the error stopped. It went away commenting out the line .configure(rest()) -- what does that line do?

[dougwilson]

Tracing it through, .configure(fn) simply invokes fn with this set to the same context, so similar to rest().call(app). The rest() call is the function exported by the @feathersjs/express module, which looks like it is decorating an Express app with some extra functionality, but then the express app is immediately being invoked by that .configure call without providing a request and response object, which eventually leads down to this module getting constructed without a request and response object and then invoked.

Pretty much boils down to the following happening:

$ node -pe 'require("express")()()'
node_modules\finalhandler\index.js:239
  return typeof res.headersSent !== 'boolean'
                   ^

TypeError: Cannot read property 'headersSent' of undefined
    at headersSent (node_modules\finalhandler\index.js:239:20)

Notice that double function call. That second function call shouldn't be there, so you just need to get that sorted out and the issue should go away 👍 I'm not familiar with how feathers is working or how to put everything together, but that is what is causing the error, which is just some logic errors somewhere in the code that eventually calls into this module without providing any arguments.

[stratosgear]

Wow plenty of things to work with. Let me see what I can do here...

Thanks for the investigative work!

[dougwilson]

No problem! I'll look into adding a guard when constructing the finalhander to validate that at least some object was provided for req and res to get a better error, though the error would have just been different in this case rather than there being no error. Since the constructor is called with every request, I just need to check what the impact of adding that check is.