Investigate `--coverage` support

[nicolo-ribaudo]

It's possible that this is already working, because Babel uses this runner and collects a coverage when running it's tests: https://github.com/babel/babel/runs/5789483528. If it already works great, I just need to document it.

It's also possible that it's only working partially, or that it's not working at all and that Babel's coverage reporting is based on something else.

(cc @psychobolt - jestjs/jest#9430 (comment))

[SimenB]

I would assume coverage doesn't work, but I've been wrong before! 😀 However, adding support for v8 based coverage should be straight forward. E.g. use https://github.com/SimenB/collect-v8-coverage (what Jest uses) and store it as v8Coverage on the test result: https://github.com/facebook/jest/blob/80cf80055682919251b9567ef482755d5f060d47/packages/jest-runner/src/runTest.ts#L338.

[nicolo-ribaudo]

Thank you!

[fisker]

I don't think the coverage works, I added c8 when using in Prettier, https://github.com/prettier/prettier/blob/3cbee742a792413211eea1e4fe2d0dad26ff37ea/.github/workflows/dev-test.yml#L79

It will be great if we can make it work.

[psychobolt]

Yes I assumed --coverageProvider=v8 would work. However, that seems to not work as well.

[SimenB]

Yeah, runner needs to hook it up. But should as mentioned be very little code 🤞

[fisker]

This CI log maybe useful, I got an error message here

Details

ERROR: Cannot assign to read only property 'message' of object 'SyntaxError: import.meta may appear only with 'sourceType: "module"' (293:39)
Consider renaming the file to '.mjs', or setting sourceType:module or sourceType:unambiguous in your Babel config for this file.'
STACK: TypeError: Cannot assign to read only property 'message' of object 'SyntaxError: import.meta may appear only with 'sourceType: "module"' (293:39)
Consider renaming the file to '.mjs', or setting sourceType:module or sourceType:unambiguous in your Babel config for this file.'
    at parser (/home/runner/work/prettier/prettier/node_modules/@babel/core/lib/parser/index.js:87:21)
    at parser.next (<anonymous>)
    at parse (/home/runner/work/prettier/prettier/node_modules/@babel/core/lib/parse.js:31:37)
    at parse.next (<anonymous>)
    at evaluateSync (/home/runner/work/prettier/prettier/node_modules/gensync/index.js:251:28)
    at sync (/home/runner/work/prettier/prettier/node_modules/gensync/index.js:89:14)
    at getAst (/home/runner/work/prettier/prettier/node_modules/istanbul-lib-instrument/src/read-coverage.js:16:12)
    at readInitialCoverage (/home/runner/work/prettier/prettier/node_modules/istanbul-lib-instrument/src/read-coverage.js:31:17)
    at generateEmptyCoverage (/home/runner/work/prettier/prettier/node_modules/@jest/reporters/build/generateEmptyCoverage.js:155:72)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)

[fisker]

And the error disappear when add coverageProvider: 'v8', log

[nicolo-ribaudo]

Yeah, the error is probably caused by istanbul/nyc reading coverage data that does not exist (I'm still wondering how Babel's coverage currently works 🤷)

EDIT I didn't see you were using c8.

[fisker]

I wounder why this error been swallowed? #6 (comment)

[SimenB]

See #13, I think that's working 🙂

[ticup]

Hi all, this might be obvious for some, but maybe not for others, so I'm leaving it here.

The jest-light-runner decreased our running time by almost a 5-fold, which is a tremendous performance increase, thanks! 🎉

We have a big typescript repo that's compiled to js using tsc and we couldn't get the coverage to work trying the proposed on-the-fly babel-register-esm loader option.
Having installed the required packages, created the babel config and running:

NODE_OPTIONS='--loader=babel-register-esm' jest --collectCoverage

We still get no coverage, even though the loader is clearly running:

As an alternative, we found we can use the v8 instrumentation and c8 outside of jest as follows:

npx c8 npx jest

Our jest config looks something like this (in its essence):

{
  runner: "jest-light-runner",
  testEnvironment: 'node',
  testMatch: ["**/?(*.)+(unit.spec).js"],
  roots: ["<rootDir>/dist"]
}

And a .c8rc.json:

{
  "src": "/src",
  "exclude": [
    "**/node_modules/**"
  ],
  "all": true,
  "extension": ".js"
}

Using v8 coverage does come with it's downsides though: jestjs/jest#11188
but this gives us an option for now.