app.js

// this sets the background color of the master UIView (when there are no windows/tab groups on it)
Titanium.UI.setBackgroundColor('#000');

var Common = require('Common');
var testModule = new Common('Hello, world!');

var win1 = Titanium.UI.createWindow({  
    title:'Tab 1',
    backgroundColor:'#fff'
});

var label1 = Titanium.UI.createLabel({
	color:'#999',
	text:testModule.doIt(),
	font:{fontSize:20,fontFamily:'Helvetica Neue'},
	textAlign:'center',
	width:'auto'
});

win1.add(label1);

// open tab group
win1.open();

Common.js

function Common(input) {
	this.phrase = input;
};

Common.prototype.doIt = function() {
	return this.phrase;
};

module.exports = Common;