test_runner: lcov reporter for test coverage

[philnash]

What is the problem this feature will solve?

We can now produce code coverage reports from the built-in test runner and (currently experimental) code coverage. Being able to report that coverage to third-parties, like SonarQube, SonarCloud, Coveralls, or Code Climate is useful to track coverage over time.

Each of these tools accepts lcov as a format for providing test coverage results.

What is the feature you are proposing to solve the problem?

I propose to write a test reporter that outputs code coverage results in the lcov format.

I've actually got quite a long way through this, as part of adding more information to the test:coverage event sent to reporters. I don't wish the reading of the lcov documentation on anyone else.

I did want to raise a feature request to see if there were any comments or thoughts before I open a PR.

What alternatives have you considered?

This could be built as a third-party module, but given lcov's seeming ubiquity as the code coverage format of choice for different code quality tools, it would be useful for users to have it built in.

In comparison with other platforms, Deno can generate an lcov report from its coverage output and Bun has an open issue to implement lcov, so this is becoming par for the course for a test runner.

I am happy to work on this. Full disclosure, I work at Sonar and this would be beneficial to our users if they decide to choose to use the Node test runner in their projects.

[MoLow]

I think this is a good idea, my only concern is source map support

[gabriel-peracio]

VSCode, as well as many extensions (e.g. ryanluker.vscode-coverage-gutters) also need lcov reports in order to annotate files. Same for bots that post coverage results to PRs.

I support and want this.

[philnash]

I think this is a good idea, my only concern is source map support

That's certainly important. But should that be handled before the test:coverage event is fired? That is, should the data that comes in the test:coverage event already have been augmented with data from the source map? It shouldn't be on each reporter to do that work, right?

And if that is the case, a reporter can be built independently of source map support. And the feature only graduates from experimental when it has source map support.

[MoLow]

my comment wasn't made to block or object to this feature - just to raise this as something to think about before the feature is implemented. if it is possible to map lcov format after it is generated to its source that works as well

[philnash]

Thanks @MoLow, I will certainly keep it in mind.

[fernandopasik]

This is a fantastic feature to have in node!

As a workaround I've used this for now
https://github.com/bcoe/c8

NODE_V8_COVERAGE=./coverage c8 -r html node --test --experimental-test-coverage src/**/*.spec.ts