How to debug child Node.JS process in Visual Studio Code?

Look for this npm module child-process-debug.

I created 2 separate launch configurations in vscode:

One for master process, other for child process

   {
        "name": "Attach",
        "type": "node",
        "request": "attach",
        "port": 5858,
        "address": "localhost",
        "restart": false,
        "sourceMaps": false,
        "outFiles": [],
        "localRoot": "${workspaceRoot}",
        "remoteRoot": null
    },
    {
        "name": "Attach child",
        "type": "node",
        "request": "attach",
        "port": 5859,
        "address": "localhost",
        "restart": false,
        "sourceMaps": false,
        "outFiles": [],
        "localRoot": "${workspaceRoot}",
        "remoteRoot": null
    }

Workflow as follows:

1. Start master node process with --debug command line switch $ node --debug master.js
2. Attach to master.js node process using Attach via debug panel
3. Place break point in the child.js process
4. Quickly detach from main process and attach to child process using Attach child

Fro debugging purposes, you may delay message sending between processes using setTimeout

// master.js
var child = child_process.fork(__dirname + './child.js')
setTimeout(function() {
    child.send('...')
}, 5000)

Make this change in your launch.json, "autoAttachChildProcesses": true

In your launch configuration add "autoAttachChildProcesses": true like shown below

{
  "type": "node",
  "request": "launch",
  "name": "Launch Program",
  "autoAttachChildProcesses": true,
  "program": "${workspaceFolder}/index.js"
}

You can easily add a new launch configuration to launch.json that allows you to attach to a running node instance with a specific port:

{
        "name": "Attach to Node",
        "type": "node",
        "address": "localhost",
        "port": 5870,
}

Just make sure you fork/spawn your node process with the --debug or --debug-brk argument.