Implement tests for optional JS helper

[jh3y]

Issue summary

There are currently no tests in place for the optional JS helper.

Expected behavior

It would be ideal to have tests in place that ensure attributes/classes are applied/removed when necessary using particular methods. Also, the ability to test that callbacks are being properly applied.

Requirements

• Create testing bundle config for webpack.
• Use mocha and chai along with js-dom for testing.

@jh3y