Karma coverage empty (no data) with Rails/sprockets

Hi,

Karma seems to work fine running the tests but I can't get any coverage output. I am using a Rails app with yarn installed modules, sprockets (asset pipeline) and karma + phantomjs.

Through Google (and [this 4 year old article](https://danott.co/posts/rails-javascript-testing-using-karma.html)) I came up with the following karma.conf.js:

```js
module.exports = function(config) {
config.set({
// base path that will be used to resolve all patterns (eg. files, exclude)
basePath: '.',

// frameworks to use
// available frameworks: https://npmjs.org/browse/keyword/karma-adapter
frameworks: ['jasmine'],

// // list of files / patterns to load in the browser
files: [
'http://localhost:3000/assets/application.js',
{
pattern: 'app/assets/javascripts/**/*.js',
watched: true,
included: false,
served: false
},
'test/javascripts/**/*_test.js'
],

// list of files to exclude
exclude: [
],

// preprocess matching files before serving them to the browser
// available preprocessors:
// https://npmjs.org/browse/keyword/karma-preprocessor
preprocessors: {
// source files, that you wanna generate coverage for
// do not include tests or libraries
// (these files will be instrumented by Istanbul)
'http://localhost:3000/assets/application.js': ['coverage'],
'app/assets/javascripts/**/*.js': ['coverage']
},

// test results reporter to use
// possible values: 'dots', 'progress'
// available reporters: https://npmjs.org/browse/keyword/karma-reporter
reporters: ['dots', 'coverage'],

// web server port
port: 9876,

// enable / disable colors in the output (reporters and logs)
colors: true,

// level of logging
// possible values: config.LOG_DISABLE || config.LOG_ERROR ||
// config.LOG_WARN || config.LOG_INFO || config.LOG_DEBUG
logLevel: config.LOG_DEBUG,

// enable / disable watching file and executing tests whenever
// any file changes
autoWatch: true,

// start these browsers
// available browser launchers:
// https://npmjs.org/browse/keyword/karma-launcher
browsers: ['PhantomJS'],

// Continuous Integration mode
// if true, Karma captures browsers, runs the tests and exits
singleRun: false,

// Concurrency level
// how many browser should be started simultaneous
concurrency: Infinity,

coverageReporter: {
dir: 'coverage-js',
reporters: [
{ type: 'html', subdir: 'default' },
{ type: 'text' }
]
}
})
}
```

I've tried with both:

```js
preprocessors: {
'app/assets/javascripts/**/*.js': ['coverage']
}
```

and

```js
preprocessors: {
'http://localhost:3000/assets/application.js': ['coverage'],
}
```

but nothing. Even both at the same time.

application.js is the file that Rails bundles with all the JS dependencies and which karma picks up fine to run the tests.

```
17 08 2017 09:47:06.703:WARN [karma]: No captured browser, open http://localhost:9876/
17 08 2017 09:47:06.720:INFO [karma]: Karma v1.7.0 server started at http://0.0.0.0:9876/
17 08 2017 09:47:06.721:INFO [launcher]: Launching browser PhantomJS with unlimited concurrency
17 08 2017 09:47:06.728:INFO [launcher]: Starting browser PhantomJS
17 08 2017 09:47:07.915:INFO [PhantomJS 2.1.1 (Mac OS X 0.0.0)]: Connected on socket bIcFbM8QsDpIbNgjAAAA with id 52351923
........
PhantomJS 2.1.1 (Mac OS X 0.0.0): Executed 8 of 8 SUCCESS (0.015 secs / 0.029 secs)
----------|----------|----------|----------|----------|----------------|
File | % Stmts | % Branch | % Funcs | % Lines |Uncovered Lines |
----------|----------|----------|----------|----------|----------------|
----------|----------|----------|----------|----------|----------------|
All files | 100 | 100 | 100 | 100 | |
----------|----------|----------|----------|----------|----------------|
```


As you can see from the debug output, the preprocessor is actually seeing the project files:

```
17 08 2017 09:49:03.018:DEBUG [plugin]: Loading karma-* from /project/node_modules
17 08 2017 09:49:03.023:DEBUG [plugin]: Loading plugin /project/node_modules/karma-coverage.
17 08 2017 09:49:03.234:DEBUG [plugin]: Loading plugin /project/node_modules/karma-jasmine.
17 08 2017 09:49:03.235:DEBUG [plugin]: Loading plugin /project/node_modules/karma-phantomjs-launcher.
17 08 2017 09:49:03.240:DEBUG [plugin]: Loading plugin /project/node_modules/karma-sprockets-mincer.
17 08 2017 09:49:03.487:DEBUG [web-server]: Instantiating middleware
17 08 2017 09:49:03.496:DEBUG [reporter]: Trying to load reporter: coverage
17 08 2017 09:49:03.497:DEBUG [reporter]: Trying to load color-version of reporter: coverage (coverage_color)
17 08 2017 09:49:03.497:DEBUG [reporter]: Couldn't load color-version.
17 08 2017 09:49:03.536:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/activeadmin/jsoneditor.js".
17 08 2017 09:49:03.560:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/activeadmin/selectize.js".
17 08 2017 09:49:03.562:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/analytics.js".
17 08 2017 09:49:03.568:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/application.js".
17 08 2017 09:49:03.571:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/analytics/app_versions.js".
17 08 2017 09:49:03.578:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/analytics/geo.js".
17 08 2017 09:49:03.581:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/base.js".
17 08 2017 09:49:03.583:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/bee.js".
17 08 2017 09:49:03.606:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/data_table.js".
17 08 2017 09:49:03.613:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/date_time_picker.js".
17 08 2017 09:49:03.617:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/notification_templates/image_crop.js".
17 08 2017 09:49:03.623:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/notification_templates/image_upload.js".
17 08 2017 09:49:03.635:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/notification_templates/landing_page.js".
17 08 2017 09:49:03.642:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/notification_templates/main.js".
17 08 2017 09:49:03.660:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/notification_templates/map.js".
17 08 2017 09:49:03.670:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/notification_templates/preview.js".
17 08 2017 09:49:03.699:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/notification_templates/query_builder.js".
17 08 2017 09:49:03.709:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/notification_templates/static_list.js".
17 08 2017 09:49:03.713:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/notification_templates/utils.js".
17 08 2017 09:49:03.715:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/business/utils.js".
17 08 2017 09:49:03.718:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/devices.js".
17 08 2017 09:49:03.720:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/landing_pages/edit.js".
17 08 2017 09:49:03.722:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/landing_pages/index.js".
17 08 2017 09:49:03.725:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/landing_pages/new.js".
17 08 2017 09:49:03.727:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/main.js".
17 08 2017 09:49:03.728:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/notification_template_landing_page.js".
17 08 2017 09:49:03.730:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/notification_templates/index.js".
17 08 2017 09:49:03.732:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/notification_templates/new_create.js".
17 08 2017 09:49:03.733:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/push_notifications/index.js".
17 08 2017 09:49:03.736:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/push_notifications/new_edit_create.js".
17 08 2017 09:49:03.738:DEBUG [preprocessor.coverage]: Processing "/project/app/assets/javascripts/touch_points.js".
17 08 2017 09:49:03.742:DEBUG [karma]: List of files has changed, trying to execute
17 08 2017 09:49:03.742:WARN [karma]: No captured browser, open http://localhost:9876/
17 08 2017 09:49:03.746:DEBUG [watcher]: Watching "/project/app/assets/javascripts"
17 08 2017 09:49:03.746:DEBUG [watcher]: Watching "/project/test/javascripts"
17 08 2017 09:49:03.756:INFO [karma]: Karma v1.7.0 server started at http://0.0.0.0:9876/
17 08 2017 09:49:03.760:INFO [launcher]: Launching browser PhantomJS with unlimited concurrency
17 08 2017 09:49:03.782:INFO [launcher]: Starting browser PhantomJS
17 08 2017 09:49:03.782:DEBUG [temp-dir]: Creating temp dir at /var/folders/qc/c_sksnns54q13ympksl8x4h00000gp/T/karma-12582815
17 08 2017 09:49:03.786:DEBUG [launcher]: /usr/local/bin/phantomjs /var/folders/qc/c_sksnns54q13ympksl8x4h00000gp/T/karma-12582815/capture.js
17 08 2017 09:49:04.985:DEBUG [web-server]: serving: /project/node_modules/karma/static/client.html
17 08 2017 09:49:04.995:DEBUG [web-server]: serving: /project/node_modules/karma/static/karma.js
17 08 2017 09:49:05.051:DEBUG [karma]: A browser has connected on socket 6nWs9u3NKcvOYYWkAAAA
17 08 2017 09:49:05.070:DEBUG [web-server]: upgrade /socket.io/?EIO=3&transport=websocket&sid=6nWs9u3NKcvOYYWkAAAA
17 08 2017 09:49:05.097:INFO [PhantomJS 2.1.1 (Mac OS X 0.0.0)]: Connected on socket 6nWs9u3NKcvOYYWkAAAA with id 12582815
17 08 2017 09:49:05.097:DEBUG [launcher]: PhantomJS (id 12582815) captured in 1.336 secs
17 08 2017 09:49:05.098:DEBUG [karma]: All browsers are ready, executing
17 08 2017 09:49:05.098:DEBUG [karma]: Captured 1 browsers
17 08 2017 09:49:05.102:DEBUG [phantomjs.launcher]:

17 08 2017 09:49:05.104:DEBUG [middleware:karma]: custom files null null null
17 08 2017 09:49:05.104:DEBUG [middleware:karma]: Serving static request /context.html
17 08 2017 09:49:05.106:DEBUG [web-server]: serving: /project/node_modules/karma/static/context.html
17 08 2017 09:49:05.109:DEBUG [middleware:source-files]: Requesting /base/node_modules/jasmine-core/lib/jasmine-core/jasmine.js?a462fdca9c003f57f07430376abd0e8850056f99 /
17 08 2017 09:49:05.110:DEBUG [middleware:source-files]: Fetching /project/node_modules/jasmine-core/lib/jasmine-core/jasmine.js
17 08 2017 09:49:05.111:DEBUG [web-server]: serving (cached): /project/node_modules/jasmine-core/lib/jasmine-core/jasmine.js
17 08 2017 09:49:05.113:DEBUG [middleware:source-files]: Requesting /base/node_modules/karma-jasmine/lib/boot.js?945a38bf4e45ad2770eb94868231905a04a0bd3e /
17 08 2017 09:49:05.117:DEBUG [middleware:source-files]: Fetching /project/node_modules/karma-jasmine/lib/boot.js
17 08 2017 09:49:05.117:DEBUG [middleware:source-files]: Requesting /base/node_modules/karma-jasmine/lib/adapter.js?7a813cc290d592e664331c573a1a796192cdd1ad /
17 08 2017 09:49:05.117:DEBUG [middleware:source-files]: Fetching /project/node_modules/karma-jasmine/lib/adapter.js
17 08 2017 09:49:05.118:DEBUG [middleware:source-files]: Requesting /base/test/javascripts/business/utils_test.js?34c8090e5c6dfc9b61c71a496b9af750aae4ae9a /
17 08 2017 09:49:05.118:DEBUG [middleware:source-files]: Fetching /project/test/javascripts/business/utils_test.js
17 08 2017 09:49:05.118:DEBUG [web-server]: serving (cached): /project/node_modules/karma-jasmine/lib/boot.js
17 08 2017 09:49:05.119:DEBUG [web-server]: serving (cached): /project/node_modules/karma-jasmine/lib/adapter.js
17 08 2017 09:49:05.120:DEBUG [web-server]: serving (cached): /project/test/javascripts/business/utils_test.js
17 08 2017 09:49:05.121:DEBUG [web-server]: serving: /project/node_modules/karma/static/context.js
........
PhantomJS 2.1.1 (Mac OS X 0.0.0): Executed 8 of 8 SUCCESS (0.015 secs / 0.025 secs)
----------|----------|----------|----------|----------|----------------|
File | % Stmts | % Branch | % Funcs | % Lines |Uncovered Lines |
----------|----------|----------|----------|----------|----------------|
----------|----------|----------|----------|----------|----------------|
All files | 100 | 100 | 100 | 100 | |
----------|----------|----------|----------|----------|----------------|

17 08 2017 09:49:05.469:DEBUG [coverage]: Writing coverage to /project/coverage-js/default
```

I had a look in https://github.com/karma-runner/karma-coverage/issues/87 but I couldn't come up with a solution.

These are my package.json devDependencies (the app is a regular web site with jQuery and some JS business logic)

```json
"devDependencies": {
"eslint": "^4.4.1",
"hot-builder": "^1.0.4",
"istanbul": "^0.4.5",
"jasmine-core": "^2.7.0",
"karma": "^1.7.0",
"karma-coverage": "^1.1.1",
"karma-jasmine": "1.1.0",
"karma-phantomjs-launcher": "^1.0.4"
},
```

Any ideas?

Thank you!