Demo getting coverage for JavaScript code

[benthorner]

The following steps show how we can get an estimate of coverage for
JavaScript (JS) code. Unfortunately it's not possible to do this
easily due to the fact that we execute all the tests for JavaScript
via Ruby code, in order to accommodate the Rails Asset Pipeline.

Instrument

First, we need to manually instrument all the JS code in the app,
so that it will log coverage stats when we run it.

Note that we can't instrument the top-level application.js file,
as this causes the test runner to go into an infinite loop (for
some reason!). Hopefully we have very little JS code in this file,
or it should be easy to move it elsewhere temporarily.

mv app/assets/javascripts{,.bak}

npx istanbul instrument app/assets/javascripts.bak -o app/assets/javascripts

mv app/assets/javascripts.bak/application.js app/assets/javascripts/

Run tests

We need to run these in the browser, so that we can extract the
coverage data later on.

govuk-docker run -p 8888:8888 finder-frontend-lite bundle exec rake jasmine

Now go to localhost:8888 and watch the tests run to completion.

Dump stats

Run the following in your browser console, and manually copy the
output into a JSON file e.g. "coverage/coverage.json".

JSON.stringify(window.__coverage__)

Generate report

Run the following to generate final report in "coverage/lcov-report/".

npx istanbul report --include coverage/coverage.json

[benthorner]

Based on the method documented here, this repo has ~90% JavaScript code coverage.