gistfile1.js

file1.js ======================
console.log('file1 evaluated');

var up = 0

module.exports = {
	up: function(){
		return ++up;
	},
	value: function(){
		return up;
	}
};


file2.js =====================
const file1 = require('./file1');
console.log( 'file2.js', file1.up())
console.log( 'file2.js', file1.value())

app.js =======================
const file1 = require('./file1');
console.log( 'app.js', file1.up())
console.log( 'app.js', file1.value())
require('./file2')
console.log( 'app.js', file1.value())


OUTPUT ======================
file1 evaluated
app.js 1
app.js 1
file2.js 2
file2.js 2
app.js 2