Node.js: How to attach to a running process and to debug the server with a console?

Question

I use 'forever' to run my application. I want to attach to the running environment to inspect my application. So what can I do?

Answer 1

From http://nodejs.org/api/debugger.html:

Advanced Usage

The V8 debugger can be enabled and accessed either by starting Node with the --debug command-line flag or by signaling an existing Node process with SIGUSR1.

Find the PID of your node process and then sending SIGUSR1 should do the trick:

kill -s SIGUSR1 nodejs-pid

Then run node-inspector and browse to the URL it indicates. More in this tutorial.

Answer 2

Starting from Node 6.3, node has a built-in debugger that can be triggered (even in a production app) by doing:

kill -USR1 <node-pid>

The node process will spit out something like this:

Debugger listening on ws://127.0.0.1:9229/f3f6f226-7dbc-4009-95fa-d516ba132fbd
For help see https://nodejs.org/en/docs/inspector

• If you can access the server from a browser, you can use chrome://inspect on http://host.domain:9229.

• If you cannot connect via a browser (e.g. the server is in a firewalled production cluster), you can activate a REPL to inspect over the command line:

  node inspect -p <node-pid>
  

• If you can't access the server from a browser, but you can SSH into that server, then setup SSH port forwarding (ssh -nNTL 9229:localhost:9229 <username>@<your_host> -i <keyfile>.pem) and you'll find your script under chrome://inspect after a few seconds.

Prior to this version, node-inspector was a separate tool for debugging Node processes. However, as documented on its own page, it is mostly deprecated as the now-bundled debugger is actively maintained and provides more advanced features. For more information on this change, see this thread.

Answer 3

Windows users

/edit 2022

This is what I use currently, configured in settings.json. This will prompt you to select a process and it will send the debug signal to it automatically. Read more below to understand what's happening.

  "launch": {
    "version": "0.2.0",
    "configurations": [
      {
        "name": "Attach to Process (Pick)",
        "type": "node",
        "request": "attach",
        "processId": "${command:PickProcess}",
        "port": 9229,
      }
    ]
  },

---

Previously

If you are on Windows that doesn't support POSIX signals, you can use this workaround from another cmd.

node -e "process._debugProcess(PID)"

---

For a detailed guide, or to set up debugging in VSCode, follow these simple steps:

1. In VSCode, open launch.json configuration or create new by clicking on the wheel
   ^{(this is the debug view CtrlShiftD)}

2. The node will listen on port 9229 by default, so add this configuration:

{
  "type": "node",
  "request": "attach",
  "name": "Attach to 9229",
  "port": 9229
},

3. Open Task Manager and locate the PID of your node process
   I could identify my by the "build" folder where the index.js is.
4. open another cmd or git-bash and run this command,
   where 21392 is the PID of your process.

node -e "process._debugProcess(21392)"

5. you should see this
6. Start debugging from VSCode Attach to 9229
   

Everything should be ready now.

Answer 4

For me, running node version 6.9.10 I had to:

kill -USR1 <node-pid>

then

node debug -p <node-pid>

the node inspect -p <node-pid> command failed for this version of node.

Answer 5

You can add a REPL to your app. For example, if you add a REPL to listen on localhost port 5001, you start your app as usual and login with telnet: telnet localhost 5001. That will take you to a prompt where you can interact with your app directly.

Alternatively, if you need to your app to "pause" when it reaches a certain state, you need to add "debugger;" lines to areas of your code where you want those breakpoints, then start the app in debug mode.

Hope that helps.

Answer 6

Even it's an old yet answered question, there is an easier way, which is passing parameters to node:

forever start -c 'node --debug-brk' main.js

If you don't want to wait for debugger to be attached, replace --debug-brk by --debug

Answer 7

To inspect nodejs and debug it, use this command

forever -w -c 'node --inspect=IP:PORT' file.js

• -c if for a custom command
• use -w to reload if the file is re-save
• You can pass ip and port for external inspect
• port custom is 9229