app.js

var os = require('os').os;

//branch logic based on platform - saves you an if statement
os(function() {
    alert('do this on android');
}, function() {
    alert('do this on iOS');
});

var platformSpecificValue = os('android string','ios string');

os.js

var osname = Ti.Platform.osname; //grab this once

function os(androidValue,iosValue) {
	if (osname === 'android') {
		return (typeof androidValue === 'function') ? androidValue() : androidValue;
	}
	else {
		return (typeof iosValue === 'function') ? iosValue() : iosValue;
	}
}

exports.os = os;