Wednesday, October 5, 2011

Writing better 3rd party Javascript with Coffeescript, Jasmine, PhantomJS and Dependence.js

Here at Bizo we recently underwent a major change to our Javascript tags that’s used in our free analytics product ( Our code gets loaded by millions of visitors each month across thousands of web sites; so our Javascript has to run reliably in just about any browser on any page.

The Old Javascript:

Unfortunately our codebase had accumulated about three years of cruft,
resulting in a single monolithic Javascript file. The file contained
a single closure, over 600 lines long, with all kinds of edge cases,
some of which no longer existed. Since you can’t access anything in a
closure from the outside, writing unit tests was nearly impossible and
the code had suffered as a result.

That’s not to say we had no tests – there were several selenium tests
that tested functionality at a high level. The problem however was
making the required changes was going to be a time consuming (and
somewhat terrifying) process. The selenium tests provided a very slow
testing feedback loop and debugging a large closure comes with it’s
own challenges. If it’s scary changing your production code, then
you’re doing something wrong.

Modularity and Dependency Management

So we decided to do a complete overhaul and rewrite our Javascript
tags in CoffeeScript (smaller code base with clearer code). The
biggest problem the original code had was that it wasn’t modular and
thus difficult to test. Ideally we wanted to split our project into
multiple files that we could unit test. To do this we needed some
kind of dependency management system for Javascript, which in 2011
surprisingly isn’t standardized yet. We looked at several projects but
none of them really met our needs. Our users are quite sensitive about
the number of http requests 3rd party Javacript makes so solutions
that load several scripts in parallel weren’t an option (ex.
Requirejs). Others like Sprockets were close but didn’t quite support
everything we needed.

We ended up writing Dependence.js, a gem to manage our Javascript
dependencies. Dependence will compile all your files in a module into
a single file. Features include javascript and/or Coffeescript
compilation, dependency resolution (via a topological sort your
dependency graph), allowing you to use an “exports” object for your
modules interface, and optional compression using Google’s Closure
compiler. Check it out on github:

Fast Unit testing with Phantom.js

Another way we were looking to improve our Javascript setup was to
have a comprehensive suite of unit tests. After looking at several
possibilities we settled on using the Jasmine test framework
( in conjunction with PhantomJS (a
headless webkit browser). So far using Jasmine and PhantomJs together
has been awesome. As our Javascript is inherently DOM coupled, each of
our unit tests executes in a separate iframe (so each test has its own
separate document object). 126 unit tests later the entire suite runs
locally in about 0.1 seconds!

Functional Testing with Selenium

Our functional tests are still executed with Selenium webdriver.
Although there are alternative options such as HtmlUnit, we wanted to
test our code in real browsers and for this Selenium is still the best
option around. A combination of capybara and rspec make for writing
functional tests with a nicer api than then raw selenium. A bonus is
that capybara allows you to swap out selenium in favor of another
driver should we ever want to switch to something else. Lastly a
custom gem for creating static html fixtures allows us to
programmatically generate test pages for each possible configuration
option found in our Javascript module. You can find that here:

Wrapping up

The new code is far more modular, comprehensively tested and way
easier to extend. Overall working with Dependence.js, CoffeeScript,
PhantomJs, Capybara, Rspec and Selenium has been a workflow that works
great for us. If you have a different workflow that you like for
Javascript projects, let us know!

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