var debug = require('debug')('express-locallibrary-tutorial:server'); I'd like to know mean of it. I'd like to know mean of it

I'm learning Express for backend development.
So I built express-locallibrary-tutorial from GitHub.
But I don't understand

var debug = require('debug')('express-locallibrary-tutorial:server');

So I hope someone will expand me in detail about it.

I found "debug": "~2.6.9" as dependency in package.json.
So I understood

var debug = require('debug')

as dependency.
But I can't understand

var debug = require('debug')('express-locallibrary-tutorial:server')

require('debug') returns a function. This is calling that function and setting debug to the value it returns. It's equivalent to:

var debug_func = require('debug');
var debug = debug_func('express-locallibrary-tutorial:server');