Coverage

[patak-dev]

@antfu creating an issue to be able to leave some links here. I won't be able to implement this myself, but I checked a bit what is needed and maybe some of these notes end up being useful.

Jest implements coverage using babel, but it can use v8 as an experimental provide. Given that native v8 code coverage is a lot faster, and that in newer versions of node it should work well, I think that vitest should use native v8 coverage directly.

This is the PR where v8 coverage was added to Jest jestjs/jest#8596

There are a few libs that were created as part of this process

• https://www.npmjs.com/package/collect-v8-coverage is a simple wrapper for the v8 coverage session. It is small enough, but maybe directly having a similar wrapper in vitest is better
• https://github.com/istanbuljs/v8-to-istanbul allows conversion of the v8 format to Istanbul, so you can use Istanbul reporters and get a nice text summary or complete HTML report.
• Looks like in Jest, they use collect-v8-coverage to create a V8Coverage for every task. Then they convert them with v8-to-istanbul (that requires the source of the file + sourcemaps).

There seems to be a merging needed of the generated reports, and there are a few libs to merge both the V8 format and the Istanbul format. I wonder if there is a way to generate the report without bringing Istanbul here

[patak-dev]

Probably a better place to look at is the web test runner coverage implementation https://github.com/modernweb-dev/web/blob/master/packages/test-runner-coverage-v8/src/index.ts, also doing what is described above