I'm working on a node project that I need to debug, but I didn't start it with the --inspect flag. Moreover, I'm using nodemon to restart node (which makes this process a little trickier).

READER DISCOUNTSave $50 on terminal.training

I've published 38 videos for new developers, designers, UX, UI, product owners and anyone who needs to conquer the command line today.

Finding the node process id

The first task is to find the node process id (aka PID). The way I do this when running with nodemon is to:

$ ps | grep nodemon | grep -v grep
 4359 ttys001    0:00.49 node /…/bin/nodemon
18357 ttys002    0:01.40 node /…/bin/nodemon --ignore public -i views

This command is doing three things:

  1. Listing all the processes running that I've started/own
  2. I'm grepping to filter only the lines that match "nodemon"
  3. Since the grep for nodemon was running, I need to filter out the word "grep" (using grep -v …)

There's other tools that you can install like pgrep and pidof - but I tend to find this is the friendliest method to finding the PID.

Now I know the PID (18357 for nodemon (in my case I have two different instances of nodemon, so I need to be savvy to work out exactly which I'm working with), I need to look at the process tree to find the PID of the child node process (this is because nodemon will spawn your node process, and I want to debug the child process, rather than nodemon itself).

$ ps -g 18357
  PID TTY           TIME CMD
18357 ttys002    0:01.40 node /…/bin/nodemon --ignore public -i views
67827 ttys002    0:02.41 /…/bin/node lib/index.js

The -g flag on ps asks for the group of subprocesses that my main nodemon process is responsible for. So now I can see the PID of my child node process (67827) and I'm ready to enable the debugger.

Turning on the debugger on a running node process

There's two ways to switch a node process into debugging mode. Using a small node script (and this should be cross compatible for all platforms). The process object in node has a _debugProcess(PID) method (which I think is undocumented though it's been around since at least mid-2014).

I'm running the node script as an inline eval'ed script:

$ node -e 'process._debugProcess(67827)'

Now my nodemon process emits the following log:

[nodemon] restarting due to changes...
[nodemon] starting `node lib/index.js`
listening on http://localhost:3009 @ 2018-03-03T12:29:58.675Z
Debugger listening on ws://
For help see https://nodejs.org/en/docs/inspector
Debugger attached.

Turning now to Chrome devtools, I will find the green node debugging icon, then clicking on that will take me to devtools for the node process:

Node debugging icon

Alternative method

Another method to triggering the debugger is to send a SIGUSR1 signal to the PID (though I'm not entirely sure how to do this on Windows). This is done using the kill command as so:

$ kill -SIGUSR1 67827

I like this method just because it's just a little more succinct.