You'll need Visual Studio Code to do your editing and debugging in.
First you'll need to install ngrok:
brew cask install ngrok
Next check this repo out and use this as the basis for your functions project.
You'll need to configure the settings.env file. You'll need your JWT token (found by going to https://app.segment.com, and under cookies find the value for auth_token).
For the SRC_FN_NAME and DEST_FN_NAME values these will be the names for your custom functions. These functions don't need to exist, but if they do be warned: They will be overwritten.
Next, put your functions code in either dest_fn.js or src_fn.js.
When ready to begin debugging, first from the command-line, run:
npm run proxy
Wait until you see messages from the command saying:
Deployed source X
Deployed destination Y
That command will then continue to run in the back-ground (but can be quit once you've finished debugging. For future debuggin sessions, be sure to run this command again).
Now, from VSCode, go to your debug pane and press run. The debugger is now ready to receive input. You can set break-points in your functions code.
To activate the function call, go to your function in the Segment workspace, and edit the function and run it as you normally would to test. As each call is made it will trigger the local break-point for you to test.
Once you're happy the function is ready to go run:
npm run deploy