v8 code coverage breaking Vitest 4

[hugs7]

Describe the bug

Hi. I noticed in Vitest 4, if you have a React component which is defined as an inline, the coverage won't work when using v8.

If you switch to istanbul, everything is okay. This wasn't an issue for me in vitest 3. In the lcov report it shows up like this

Even though I definitely have a test for this component

Reproduction

export const SomeComponent = () => <div>Some simple component</div>

And test file

import { screen } from "@testing-library/react";

import { SomeComponent } from './SomeComponent.tsx';

describe('<SomeComponent />',  () => {
   it('should render without crashing', () => {
      render(<SomeComponent />);
      expect(screen)).toHaveTextContent("Some simple component");
   })
})

System Info

System:
    OS: Linux 6.8 Ubuntu 24.04.1 LTS 24.04.1 LTS (Noble Numbat)
    CPU: (20) x64 12th Gen Intel(R) Core(TM) i7-12700K
    Memory: 28.45 GB / 31.98 GB
    Container: No
    Shell: 5.2.21 - /bin/bash
  Binaries:
    Node: 25.0.0 - /home/hugo/.nvm/versions/node/v25.0.0/bin/node
    npm: 11.7.0 - /home/hugo/.nvm/versions/node/v25.0.0/bin/npm
  Browsers:
    Chrome: 142.0.7444.175
    Firefox: 145.0.2
    Firefox Developer Edition: 145.0.2
  npmPackages:
    @vitest/coverage-v8: ^4.0.16 => 4.0.16 
    vite: ^7.3.0 => 7.3.0 
    vitest: ^4.0.16 => 4.0.16

Used Package Manager

npm

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
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• The provided reproduction is a minimal reproducible example of the bug.

[hugs7]

If you use a component with standard return rather than shorthand, it doesn't seem to be an issue

[github-actions]

Hello @hugs7. Please provide a minimal reproduction using a GitHub repository or StackBlitz (you can also use examples). Issues marked with needs reproduction will be closed if they have no activity within 3 days.

[AriPerkkio]

Reproduction does not mention how JSX transform is done. Are you using esbuild for that? Or vite-plugin-react? Or swc?

Plese set up a reproduction. There's not much to do without one.

[hugs7]

Hey @AriPerkkio, sure I can make an example repo - give me some time.

And yes I'm using @vitejs/plugin-react-swc. Even with the non-swc it shouldn't affect coverage right?

[AriPerkkio]

SWC has had issues with coverage before too:

• Code Coverage Issue with Istanbul Provider #4185

Even when used outside Vitest:

• Code Coverage Issue with Istanbul Provider #4185 (comment)

[hugs7]

Hi @AriPerkkio, sorry if i didn't make it clear. The issue isn't with istanbul - it only happens when using v8.

Please see minimal repository with the issue here

https://github.com/hugs7/nx-vitest-4-v8-coverage-inline-example

I included readme for you to replicate the issue

[hugs7]

You were right about swc though. I created a branch feature/non-swc-react-plugin and using v8 + non-swc vite plugin. Works fine.

What is it about the combination of these packages which is breaking coverage reporting?

[AriPerkkio]

As this works with @vitest/coverage-istanbul there might be something that can be fixed on @vitest/coverage-v8.

Transpiled code, source maps and v8 script coverage for debugging

__vite_ssr_exportName__("SomeInlineComponent", () => { try { return SomeInlineComponent } catch {} });
const __vite_ssr_import_0__ = await __vite_ssr_import__("/@fs/home/node/nx-vitest-4-v8-coverage-inline-example/node_modules/react/jsx-dev-runtime.js", {"importedNames":["jsxDEV"]});

const SomeInlineComponent = ()=>/*#__PURE__*/ (0,__vite_ssr_import_0__.jsxDEV)("div", {
        "data-testid": "some-inline-component",
        children: "Some Inline Component"
    }, void 0, false, {
        fileName: "/home/node/nx-vitest-4-v8-coverage-inline-example/ui/src/features/SomeInlineComponent/SomeInlineComponent.component.tsx",
        lineNumber: 2,
        columnNumber: 3
    }, this);
//# sourceMappingSource=vite-generated
//# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJtYXBwaW5ncyI6IkFBQUEsQ0FBQTs7O0FBQU8sTUFBTUEsc0JBQXNCLHFCQUNqQyw4QkFBQ0M7UUFBSUMsZUFBWTtrQkFBd0IiLCJuYW1lcyI6WyJTb21lSW5saW5lQ29tcG9uZW50IiwiZGl2IiwiZGF0YS10ZXN0aWQiXSwiaWdub3JlTGlzdCI6W10sInNvdXJjZXMiOlsiU29tZUlubGluZUNvbXBvbmVudC5jb21wb25lbnQudHN4Il0sInNvdXJjZXNDb250ZW50IjpbImV4cG9ydCBjb25zdCBTb21lSW5saW5lQ29tcG9uZW50ID0gKCkgPT4gKFxuICA8ZGl2IGRhdGEtdGVzdGlkPVwic29tZS1pbmxpbmUtY29tcG9uZW50XCI+U29tZSBJbmxpbmUgQ29tcG9uZW50PC9kaXY+XG4pO1xuIl0sImZpbGUiOiIvc3JjL2ZlYXR1cmVzL1NvbWVJbmxpbmVDb21wb25lbnQvU29tZUlubGluZUNvbXBvbmVudC5jb21wb25lbnQudHN4In0=

{
  "version": 3,
  "mappings": ";;;AAAO,MAAMA,sBAAsB,qBACjC,8BAACC;QAAIC,eAAY;kBAAwB",
  "names": [
    "SomeInlineComponent",
    "div",
    "data-testid"
  ],
  "ignoreList": [],
  "sources": [
    "file:///home/node/nx-vitest-4-v8-coverage-inline-example/ui/src/features/SomeInlineComponent/SomeInlineComponent.component.tsx"
  ],
  "sourcesContent": [
    "export const SomeInlineComponent = () => (\n  <div data-testid=\"some-inline-component\">Some Inline Component</div>\n);\n"
  ],
  "file": "/src/features/SomeInlineComponent/SomeInlineComponent.component.tsx"
}

[
  {
    "functionName": "",
    "ranges": [
      {
        "startOffset": 0,
        "endOffset": 1957,
        "count": 1
      }
    ],
    "isBlockCoverage": true
  },
  {
    "functionName": "",
    "ranges": [
      {
        "startOffset": 13,
        "endOffset": 1957,
        "count": 1
      }
    ],
    "isBlockCoverage": true
  },
  {
    "functionName": "",
    "ranges": [
      {
        "startOffset": 256,
        "endOffset": 309,
        "count": 1
      },
      {
        "startOffset": 299,
        "endOffset": 307,
        "count": 0
      }
    ],
    "isBlockCoverage": true
  },
  {
    "functionName": "SomeInlineComponent",
    "ranges": [
      {
        "startOffset": 523,
        "endOffset": 897,
        "count": 1
      }
    ],
    "isBlockCoverage": true
  }
]

[hugs7]

Appreciate you looking into this so quickly @AriPerkkio. Let me know if I can help at all

[AriPerkkio]

@hugs7 could you re-check if this is still an issue? There was an improvement (AriPerkkio/ast-v8-to-istanbul#120) in Vitest's transitive dependency that does the coverage remapping. When using your reproduction and doing rm -rf node_modules package-lock.json; npm install, I'm seeing the component marked as covered: