First grep the heroku logs for the message that will state that the debugger service has started:
$ heroku logs -t --app=[APP_ID] | grep 'Debugger listening on'
Then in another shell enable the debugger service of the Node process by connecting through SSH and sending the process SIGUSR1:
$ heroku ps:exec --app=[APP_ID]
Establishing credentials... done
Connecting to web.1 on ⬢ [APP_ID]
$ ps aux | grep node
u46040 4 0.8 0.0 987944 38544 ? Sl 14:53 0:00 node node_modules/.bin/forever -c node --max_old_space_size=960 build/index.js
u46040 27 0.0 0.0 18168 2464 ? S 14:53 0:00 bash --login -c node_modules/.bin/forever -c "node --max_old_space_size=960" build/index.js
u46040 60 7.8 0.1 1320688 106020 ? Sl 14:53 0:07 node --max_old_space_size=960 /app/build/index.js
u46040 75 0.0 0.0 8880 772 pts/0 S+ 14:55 0:00 grep node
$ kill -s USR1 60
Back in the shell where the heroku logs are being grepped, it should now show something like:
2018-02-16T14:56:08.233521+00:00 app[web.1]: Debugger listening on ws://127.0.0.1:9229/bc540eda-30fd-4023-8474-1a8dfe4faf41
You can now kill the logging process and start forwarding the remote debugger service’s port to your local machine:
$ heroku ps:forward 9229 --app=[APP_ID]
Establishing credentials... done
SOCKSv5 proxy server started on port 1080
Listening on 9229 and forwarding to web.1:9229
Use CTRL+C to stop port fowarding
Once that’s up and running, open Chrome and navigate to chrome://inspect. Under the Remote Target heading you should now see the process, which you can start debugging by clicking the ‘Inspect’ link.
When you’re finished debugging, it’s probably a good idea to restart the heroku process, I haven’t done any inspecting of the negative impact of leaving the debugger service running, but it probably has some impact.