fix(jest-config): allow `collectCoverage` to be configured at project level #15854

hainenber · 2025-10-07T15:55:47Z

Summary

collectCoverage to be toggled on-off at project level would be pretty handy for folks in monorepo. To implement this, collectCoverage's default value for projects has to be undefined unless specified otherwise.

This only affects --showConfig CLI option as JSON spec doesn't allow undefined value to be printed out so the result will miss collectCoverage value.

One caveat from this approach is the folks specifying collectCoverage: undefined for a project would not see desired result of filtering coverage statistics for that particular project but fortunately, the accepted value type in the doc is Boolean so I suspect people will realize the issue and use collectCoverage: true | false per doc.

Test plan

Green CI
UAT passed by affected user in linked GH issue.

… level Signed-off-by: hainenber <[email protected]>

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Signed-off-by: hainenber <[email protected]>

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commit: d533273

…undefined` Signed-off-by: hainenber <[email protected]>

Signed-off-by: hainenber <[email protected]>

ahnpnl

I've tested the current behavior of 30.2.0 withcollectCoverage option. Here are what I found.

Given a config

export default {
  collectCoverage: true,
  projects: [
    {
      displayName: 'Package 1',
      rootDir: '<rootDir>/packages/package-1',
      testMatch: ['<rootDir>/test-e2e/**/*.e2e.js'],
    },
    {
      displayName: 'Package 2',
      rootDir: '<rootDir>/packages/package-2',
      testMatch: ['<rootDir>/test-e2e/**/*.e2e.js'],
    },
    {
      displayName: 'Package 3',
      rootDir: '<rootDir>/packages/package-3',
      testMatch: ['<rootDir>/test-e2e/**/*.e2e.js'],
    },
  ],
};

When having global collectCoverage: true, the collectCoverage value per project will be always set to false
When setting a collectCoverage: false for a project which means overriding the global level. This overriding value is correctly set for config argument of shouldInstrument function.

In summary

Solving the warning issue in #15588 only requires the change in jest-types, test-utils and jest-config/src/index.ts
To make the overriding value of collectCoverage works, it requires a change here

jest/packages/jest-runner/src/runTest.ts

Line 316 in 3e87145

await runtime.collectV8Coverage();

The condition collectV8Coverage currently takes value from

jest/packages/jest-runner/src/runTest.ts

Line 301 in 3e87145

const collectV8Coverage =

which never uses the overriding value of project

hainenber · 2025-10-11T10:23:33Z

Thanks @ahnpnl for the review! That would help resolving one of the pain point of changing collectCoverage type but I wonder if the proposed change can work with babel coverage provider too as well 🤔

ahnpnl · 2025-10-11T10:31:16Z

The original issue #15588 uses provider v8 so I can only propose the change for v8. I've also checked for Babel provider. It has exactly same issue like v8. However, Idk where collecting coverage is called when using Babel. I can only find in jest-runtime where collecting v8 coverage is called.

Usually, jest-runtime decides whether to collect coverage based on config and jest-transform will ensure the codes are instrumented properly to collect coverage.

I hope these information will help you further for your investigation 👍

hainenber · 2025-10-11T10:32:27Z

Thank you for your effort. I'll go back to the lab for this one.

hainenber · 2025-10-11T15:21:22Z

I've re-examined both approach and unfortunately with default collectCoverage value at project level to be false, it's nigh impossible to distinguish between user-specific falsy value and default falsy value, except for its type.

I suppose we have to compromise by letting collectCoverage to be optional for a clear separation of 2 said factions.

ahnpnl · 2025-10-11T15:26:41Z

Solving the warning issue in [Bug]: Cannot specify unique test coverage per project #15588 only requires the change in jest-types, test-utils and jest-config/src/index.ts

To make the overriding value of collectCoverage works, it requires a change here

jest/packages/jest-runner/src/runTest.ts

Line 316 in 3e87145

await runtime.collectV8Coverage();

The condition collectV8Coverage currently takes value from

jest/packages/jest-runner/src/runTest.ts

Line 301 in 3e87145

const collectV8Coverage =

which never uses the overriding value of project

did you make this change to make v8 working correctly first? I think this one we should have

Have you checked where Babel coverage is called?

hainenber · 2025-10-11T15:53:23Z

Yes, I've made changes to collectCoverage value to take the overridden value and limit only for v8 coverage provider

Changes in jest repo

Allow project-level collectCoverage to override global one

Revert changes in shouldInstrument to be on par with current Jest version

Result in test repo with jest built from above change

Have you checked where Babel coverage is called?

This is where I think the Babel coverage gets called after transformed by Babel.

ahnpnl · 2025-10-11T16:37:33Z

I've re-examined both approach and unfortunately with default collectCoverage value at project level to be false, it's nigh impossible to distinguish between user-specific falsy value and default falsy value, except for its type.

I suppose we have to compromise by letting collectCoverage to be optional for a clear separation of 2 said factions.

I got this now. Indeed if we make collectCoverage as a mandatory prop for ProjectConfig, it’s impossible to tell if users disable/enable on purpose for projects. I think optional is the only way

ahnpnl · 2025-10-12T09:05:49Z

I think we just need to adjust https://github.com/jestjs/jest/blob/main/e2e/__tests__/coverageReport.test.ts to reflect this change and all will be good 👍

fix(jest-config): allow collectCoverage to be configured at project…

9063fae

… level Signed-off-by: hainenber <[email protected]>

doc: add CHANGELOG entry

a4ba799

Signed-off-by: hainenber <[email protected]>

hainenber and others added 4 commits October 7, 2025 23:17

fix(ci): ensure project-specific collectCoverage default value is `…

56fc4cc

…undefined` Signed-off-by: hainenber <[email protected]>

fix(ci): resolve lint issues + update failed snapshot

2f17ab4

Signed-off-by: hainenber <[email protected]>

chore(ci): update failed snapshot in logDebugMessages.test.ts

7808876

Signed-off-by: hainenber <[email protected]>

Merge branch 'main' into fix/per-project-collect-coverage

d533273

ahnpnl reviewed Oct 11, 2025

View reviewed changes

hainenber marked this pull request as draft October 11, 2025 13:45

hainenber marked this pull request as ready for review October 11, 2025 15:21

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