Example module using Asynchronous Module Definition (AMD) design pattern.

AMDPattern.js

define(function() {
    var _counter = 0;
    function generateId() {
        return "customId" + _counter++;
    }

    function create(tagName, id) {
        var el = document.createElement(tagName);
        el.id = id || generateId();
        return el;
    }
    return {
        generateId : generateId,
        create : create
    }
});

To use this module you'll require using a scriptloader like RequireJS.

require(['myAMDModule'], function(myMod) {
    var el1 = myMod.create('div', 'myElement');
    var el2 = myMod.create('div');

    console.log(el1.id);
    console.log(el2.id);
});