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

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.

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.

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.