module: fix ERR_INTERNAL_ASSERTION in CJS module loading

[vivek958]

Description

Fixes #60401

The loadCJSModule function in lib/internal/modules/esm/translators.js was not validating source content before passing it to the native compileFunctionForCJSLoader function. When source was null or undefined, this caused an internal assertion failure instead of throwing a meaningful error.

Changes

• Added source validation in loadCJSModule to check for null/undefined/empty source
• Throws ERR_INVALID_RETURN_PROPERTY_VALUE with descriptive message instead of internal assertion

Testing

The validation prevents the internal assertion that was occurring with custom loaders returning null source.

• Added test file: test/parallel/test-esm-loader-null-source.js
• Tests verify proper error handling for null/undefined/empty source
• Tests ensure ERR_INTERNAL_ASSERTION is not thrown

Checklist

• make -j4 test (UNIX) or vcbuild test (Windows) passes (will be verified by CI)
• commit message follows commit guidelines

[nodejs-github-bot]

Review requested:

• @nodejs/loaders

[aduh95]

The validation prevents the internal assertion that was occurring with custom loaders returning null source. Not yet tested due to my system limitations.

Could you add a test?

[vivek958]

Could you add a test?

done, added test case in second commit to verify the fix works correctly. The test covers null, undefined, and empty string source values to ensure ERR_INVALID_RETURN_PROPERTY_VALUE is thrown instead of ERR_INTERNAL_ASSERTION. @aduh95

[vivek958]

Friendly ping @nodejs/loaders - would appreciate any feedback when you have time!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.58%. Comparing base (ba7cdf4) to head (4d2299a).
⚠️ Report is 39 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60408      +/-   ##
==========================================
+ Coverage   88.56%   88.58%   +0.01%     
==========================================
  Files         704      704              
  Lines      207774   207841      +67     
  Branches    40027    40043      +16     
==========================================
+ Hits       184020   184120     +100     
+ Misses      15800    15765      -35     
- Partials     7954     7956       +2     

Files with missing lines	Coverage Δ
lib/internal/modules/esm/translators.js	93.03% <100.00%> (+0.13%)	⬆️

... and 46 files with indirect coverage changes

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[vivek958]

@aduh95 I've made some changes, checks are needed again

[vivek958]

Friendly ping @aduh95 - would appreciate any feedback when you have time!