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Synchronized Times In Ruby & JavaScript Acceptance Tests Using Capybara, Timecop & Sinon.JS

Any good Ruby developer that tests time-dependent code has used the Timecop gem. Timecop provides dead simple time travel and freezing capabilities to Ruby's standard library. But what if you are working on a rich JavaScript application that is backed and tested by something like a Rails application and you want to alter the test browser's clock as well? The answer is pretty simple, but let's first examine all the parts at play here.

Rails & Capybara

My examples leverage a very simple Rails integration test setup using Capybara's DSL within a basic Rails integration test case. Much like Wyatt has described here and I have shown in a demo project on github. So if your integration test setup is different, transpose my code to fit your needs. Also, there are a few drivers for Capybara that support full JavaScript integration, most notably capybara-webkit. But the latest on the scene is Poltergeist which uses the badass PhantomJS project. I highly recommend you switch to this driver! That said, any Capybara driver that fully supports Capybara's #execute_script should work just fine.

Ruby & JavaScript Times

JavaScript date values represent time in milliseconds since Unix Epoch. Many 3rd-party JavaScript date libraries use millisecond integers for both instantiating and altering these objects. Ruby on the other hand has a much higher precision and thanks to ActiveSupport's core extensions to Ruby's date and time classes we can easily represent these values for JavaScript. Specifically, ActiveSupport adds a #to_i method that returns an integer which represents that date or time in seconds since Unix Epoch. In my example code, you will see that I multiple this by 1000 to get the millisecond representation.

ActiveSupport also provides an #advance core extension to all date and time classes. This method is Valuable As Fuck™ since it returns a new time instance that has moved backwards or forwards given a hash of options. For example, returning a time instance moved forward by 20 minutes would look like this @time.advance(minutes:20). See the documentation for all the options and remember, you can provide negative values to move backward. In short, the advance method is awesome!

Faking Time With Sinon.JS

Sinon.JS is a small stand-alone library that provides spies, stubs and mocks for your JavaScript. To be honest, I use Jasmine and supporting extensions for most of these features. However, Sinon.JS has one killer feature, faking time! Yup, it allows you to freeze JavaScript's clock to a specific time and tick it forward as needed. I want you to ponder the benefits of that for awhile. Imagine you have time sensitive JavaScript code that uses setInterval() or the like. Sinon.JS will actually allow you to tick time forward and still maintain compatibility with that code's behavior! Basically Sinon.JS is equal to our Ruby Timecop gem and then some! Check out their clock API or read the code if you want to learn more. Remember, it is safe to include Sinon.JS in any existing JavaScript project since it will not do anything unless you ask it too. So no fear in it clashing with your other JavaScript test setup.

Putting It All Together

So now the fun part, some code examples. First, you need to get Sinon.JS in your Rails JavaScript manifest. If you are smart, you have already setup a system where you can specify top level asset manifests for your JavaScript application per test environment. If not, you might want to take a look at two posts I previously published on setting up Jasminerice for testing Spine.JS applications.

These file snippets below assume you have an integration.js manifest which includes both a vendored Sinon.JS then a sub file which actually initializes Sinon.JS for our integration test run. In this case below, we are first processing a CoffeeScript file with ERB and then initializing Sinon.JS' fake timers to a default time. In my case, this is 8:30am central standard time. Now we can assure that Capybara's browser engine will be frozen at that time and ready to move forward for each test.

// File app/assets/javascripts/integration.js

//= require sinon-1.4.2
//= require integration/sinon
//= require application
# File app/assets/javascripts/integration/

<% central = ActiveSupport::TimeZone['Central Time (US & Canada)'] %>
<% today = central.parse('8:30am').to_i * 1000 %>

window.clock = sinon.useFakeTimers <%= today %>

Now, the Ruby code. Here is slimmed down version of my base integration test case which uses MiniTest::Spec to drive Capybara tests. The first thing I do before any integration test is use Timecop to travel to 8:30am. This means that both Ruby and JavaScript are synced to the exact millisecond in time. Any test that needs to move time forward must call the #advance_time test helper. This method takes a hash of options which is passed directly to the #advance method I previously talked about. It measures the milliseconds between now and the advancement and sends that directly to Sinon.JS' fake timers using Capybara's #execute_script method. So calling advance_time(seconds:20) in Ruby now moves time forward in both Ruby and JavaScript. Epic win!!!

# File test/test_helper_integration.rb

require "test_helper"
require "capybara/rails"

module ActionDispatch
  class IntegrationTest

  include Capybara::DSL

  before { travel '8:30am' }
  after  { Capybara.reset_sessions! }


  def travel(parseable_time)
    Timecop.return parse_time(parseable_time)

  def parse_time(time)
    time.is_a?(String) ? Chronic.parse(time) : time

  def advance_time(options)
    now_ms = Time.current.to_i * 1000 Time.current.advance(options)
    traveled_ms = (Time.current.to_i * 1000) - now_ms
    advance_sinon traveled_ms

  def advance_sinon(ms)
    page.execute_script "if (window.clock) { window.clock.tick(#{ms}); }"


I have found the following technique critical to fully testing a recent time based JavaScript application I have developed. I hope you find this technique useful as well and as always, please contribute your thoughts or questions below. Cheers!