diff --git a/package.json b/package.json
index 1c8fcdc..18d771f 100644
--- a/package.json
+++ b/package.json
@@ -4,7 +4,7 @@
   "description": "JS client for SeatGeek's Sixpack AB testing framework.",
   "main": "sixpack.js",
   "scripts": {
-    "test": "mocha"
+    "test": "mocha --globals document,window"
   },
   "repository": {
     "type": "git",
diff --git a/sixpack.js b/sixpack.js
index ee7737e..cdd0ec2 100644
--- a/sixpack.js
+++ b/sixpack.js
@@ -2,7 +2,10 @@
     // Object.assign polyfill from https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/assign#Polyfill
     Object.assign||Object.defineProperty(Object,"assign",{enumerable:!1,configurable:!0,writable:!0,value:function(e){"use strict";if(void 0===e||null===e)throw new TypeError("Cannot convert first argument to object");for(var r=Object(e),t=1;t<arguments.length;t++){var n=arguments[t];if(void 0!==n&&null!==n){n=Object(n);for(var o=Object.keys(Object(n)),a=0,c=o.length;c>a;a++){var i=o[a],b=Object.getOwnPropertyDescriptor(n,i);void 0!==b&&b.enumerable&&(r[i]=n[i])}}}return r}});
 
-    var sixpack = {
+    // check if on node
+    var on_node = typeof window === "undefined";
+
+    var sixpack = (!on_node && window.sixpack) ? window.sixpack : {
         base_url: "http://localhost:5000",
         ip_address: null,
         user_agent: null,
@@ -10,23 +13,22 @@
         persist: true,
         cookie_name: "sixpack_client_id",
         cookie_domain: null,
-        ignore_alternates_warning: null
+        ignore_alternates_warning: false
     };
-
-    // check if on node, else expose on browser's global window object
-    var on_node = false;
-    if (typeof window === "undefined") {
-        on_node = true;
-    } else {
-        window["sixpack"] = sixpack;
+    if (!on_node) {
+        window.sixpack = sixpack;
     }
 
-    sixpack.generate_client_id = function () {
+    function generate_uuidv4() {
         // from http://stackoverflow.com/questions/105034
-        var client_id = 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
+        return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
             var r = Math.random()*16|0, v = c == 'x' ? r : (r&0x3|0x8);
             return v.toString(16);
         });
+    }
+
+    sixpack.generate_client_id = function () {
+        var client_id = generate_uuidv4();
         if (!on_node && this.persist) {
             var cookie_value = this.cookie_name + "=" + client_id + "; expires=Tue, 19 Jan 2038 03:14:07 GMT; path=/";
             if (this.cookie_domain) {
@@ -163,8 +165,6 @@
         }
     };
 
-    var counter = 0;
-
     var _request = function(uri, params, timeout, callback) {
         var timed_out = false;
         var timeout_handle = setTimeout(function () {
@@ -173,7 +173,8 @@
         }, timeout);
 
         if (!on_node) {
-            var cb = "callback" + (++counter);
+            var suffix = generate_uuidv4().replace(/-/g, '');
+            var cb = "callback" + suffix;
             params.callback = "sixpack." + cb
             sixpack[cb] = function (res) {
                 if (!timed_out) {
diff --git a/test/sixpack-test.js b/test/sixpack-test.js
index 0206448..4ddf7a5 100644
--- a/test/sixpack-test.js
+++ b/test/sixpack-test.js
@@ -2,12 +2,21 @@ var mocha  = require('mocha');
 var assert = require('chai').assert;
 var expect = require('chai').expect;
 
-describe("Sixpack", function () {
+function createSixpackInstance(on_node) {
+    if (!on_node) {
+        global.document = global.document || {};
+        global.window = global.window || {};
+    }
+    delete require.cache[require.resolve('../')]
+    return require('../');
+}
+
+describe("Sixpack in node", function () {
     var sixpack;
     var session;
 
     beforeEach( () => {
-        sixpack = require('../');
+        sixpack = createSixpackInstance(true);
         session = new sixpack.Session();
 
         // Override default base_url when the SIXPACK_BASE_URL
@@ -212,3 +221,25 @@ describe("Sixpack", function () {
         });
     });
 });
+
+describe('Sixpack in browser', () => {
+    afterEach(() => {
+        delete global.document;
+        delete global.window;
+    });
+
+    it('should create sixpack instance in browser', () => {
+        global.window = {};
+        expect(window.sixpack).to.be.undefined;
+        createSixpackInstance(false);
+        expect(window.sixpack).to.be.an('object');
+    });
+
+    it('should not create another sixpack instance if exists in browser', () => {
+        var globalSixpack = {};
+        global.window = { sixpack: globalSixpack };
+        createSixpackInstance(false);
+
+        expect(window.sixpack).to.be.equal(globalSixpack);
+    });
+});