How does the VS Code debugger attach to an already running node process?

Question

How is Visual Studio Code debugger able to attach to a already running node process?

I simply used npm start to run my node application and then use the VS Code "attach to process Id" launch configuration to start the debugger. I didn't had to start my application with --inspect or --debug flag. How is this actually working?

Answer 1

As can be found in the nodejs docs:

Node.js will also start listening for debugging messages if it receives a SIGUSR1 signal. (SIGUSR1 is not available on Windows.) In Node.js 7 and earlier, this activates the legacy Debugger API. In Node.js 8 and later, it will activate the Inspector API.

VS Code sends SIGUSR1 to the process. From the VS Code docs:

the debugger tries to attach to this process after having sent a USR1 signal. With this setting, the debugger can attach to an already running process that was not started in debug mode.