gistfile1.js

// common.js ======================================

exports.foo = function() {
    return 'bar';
}

// ================================================


// anotherfile.js =================================

var common = require('common');
console.log(common.foo());

// ================================================


// when common.js is loaded, node wraps it in a function definition
// and passed in an exports object e.g.

function (exports, require, module, __filename, __dirname) { 

    exports.foo = function() { 
        return 'bar';
    };
}