Invalid coverage with `--experimental-monocart` when using `ts-node`

[AriPerkkio]

All metrics of coverage report are incorrectly set to 100% when using --experimental-monocart with ts-node.

Repro, make sure you download the project locally as WebContainers do not support V8 coverage/CDP: https://stackblitz.com/~/edit/c8-ts-node-invalid-monocart

$ pnpm test

------------|---------|----------|---------|---------|-------------------
File        | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
------------|---------|----------|---------|---------|-------------------
All files   |   76.47 |       25 |     100 |   76.47 |                   
 example.ts |   76.47 |       25 |     100 |   76.47 | 3-4,13-14         
------------|---------|----------|---------|---------|-------------------

$ pnpm test:monocart

------------|---------|----------|---------|---------|-------------------
File        | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
------------|---------|----------|---------|---------|-------------------
All files   |     100 |      100 |     100 |     100 |                   
 example.ts |     100 |      100 |     100 |     100 |                   
------------|---------|----------|---------|---------|-------------------

[AriPerkkio]

I guess this is because runtime JS files are transformed on the fly, and Monocart does not have access to those? c8 would require a loader or require hook in order to capture all files that user executed? cc. @cenfun

[cenfun]

I thought it should be the issue with the sourcemap.
I used your example and it works with mcr CLI, see here (branch is example)

npm run test

> mcr-ts-node@1.0.0 test
> cross-env NODE_OPTIONS="--loader ts-node/esm --no-warnings" npx mcr ts-node ./test/example.test.ts -o coverage-reports/ts

[MCR] Execute: ts-node ./test/example.test.ts
[MCR] Loaded: mcr.config.ts
hello
[MCR] Coverage Report
┌──────────────┬─────────┬────────────┬──────────┬───────────┬─────────┬─────────────────┐
│ Name         │   Bytes │ Statements │ Branches │ Functions │   Lines │ Uncovered Lines │
├──────────────┼─────────┼────────────┼──────────┼───────────┼─────────┼─────────────────┤
│ src          │         │            │          │           │         │                 │
│ └ example.ts │ 79.28 % │    75.00 % │  62.50 % │  100.00 % │ 53.85 % │ 2-4,12-14       │
├──────────────┼─────────┼────────────┼──────────┼───────────┼─────────┼─────────────────┤
│ Summary      │ 79.28 % │    75.00 % │  62.50 % │  100.00 % │ 53.85 % │                 │
└──────────────┴─────────┴────────────┴──────────┴───────────┴─────────┴─────────────────┘

But I haven't figured out why c8 isn't working. The differences:
1, tsconfig.json: https://github.com/cenfun/mcr-ts-node/blob/example/tsconfig.json
2, cross-env NODE_OPTIONS="--loader ts-node/esm --no-warnings"
3, package.json { "type": "module" }
4, CLI: mcr

[AriPerkkio]

That example is not using c8 at all?

[cenfun]

yes, I haven't figured out why c8 isn't working

[cenfun]

I have found that the issue is related to the generated source code.

when using c8 + ts-node, we can not get the generated source code, but there is only a fake code with lineLengths of source-map-cache, see https://nodejs.org/docs/latest/api/cli.html#source-map-cache and nodejs/node#48460
It said, the sourcemap contains the original code, but it does not have the generated code, instead, it has lineLengths only.
So Monocart cannot get the precise location by parsing the AST of the generated code.

In order to get the generated code, mcr CLI uses Node.js customization-hooks to save the generated code in some cases. (It doesn't always succeed, for example, it fails when using tsx)
see https://github.com/cenfun/monocart-coverage-reports/blob/main/lib/register/hooks.js

[AriPerkkio]

Yep, that's exactly what I meant by #561 (comment).