[feat] module api for intergrators

Im integrating code coverage into Bazel: https://github.com/bazelbuild/rules_nodejs However, rather than reimplementing the internal logic we see in this lib it would be great if there was a public facing API that could be used by downstream integrators.

For this kind of integration the main functionality needed

• gathering coverage (without child process/forking)
• producing machine readable reports (in this case we’re after lcov)
• merging reports between projects that might have overlapping coverage

A code example of the api we’re after might look like this

const c8 = require('c8');

c8.collectCoverage({reporters: ['text-summary', 'lcov']});
jasmine.execute();
const reports = await c8.completeCoverage();

const textSummaryReport = reports.find(r => r.type = 'text-summary');
console.log(textSummaryReport);

cons lcovReport = reports.find(r => r.type = 'lcov');
fs.writeFileSync('to/some/location', lcovReport);
// or maybe merge/process it in some other way

Some important points here:

• The lib should not do any IO
• The lib should not require environment variables to collect coverage

I’d be happy to contribute to this design or implementation if needed. Let me know if this is something we’d like to do with this lib

[SimenB]

I opened up https://github.com/istanbuljs/v8-to-istanbul/issues/33. By using that, and copying some istanbul-lib-* code from Report.js in this repo, I was able to make it work in Jest 🎉

The part I copied from c8:

const map = libCoverage.createCoverageMap({});

v8CoverageResult.forEach(res => map.merge(res));

const context = libReport.createContext({
  dir: globalConfig.coverageDirectory,
});

const tree = libReport.summarizers.pkg(map);

tree.visit(reports.create('html'), context);

Not too much. So I don’t think we necessarily need to put any more code in c8 - making v8-to-istanbul support not talking to the FS seems to cover it?

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The inspector API abstraction I put together: https://github.com/SimenB/jest/blob/917efc3398577c205f33c1c2f9a1aeabfaad6f7d/packages/jest-coverage/src/index.ts

Usage:

const v8CoverageInstrumenter = new CoverageInstrumenter();

await v8CoverageInstrumenter.startInstrumenting();

// do your work that you want instrumented here

const v8CoverageResult = await v8CoverageInstrumenter.stopInstrumenting();

Note that this does not at all have to live in Jest, it was just easiest for me when doing this work to have it there. It might live in the istanbul org, maybe?

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Results in this:

(this is just require('typescript'))

[bcoe]

@Toxicable @SimenB might be worth moving this feature request to Node.js? could imagine this being possible with a signal, not sure what signal.