Error in CJS module `variable exports is undeclared` when assigning to `exports`

[fingerartur]

CJS module support is bugged.

Consider this library. This library is in CJS format and the first few lines of its code look like this:

// "use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.isPlainObject = exports.clone = exports.recursive = exports.merge = exports.main = void 0;
module.exports = exports = main;
function main() { /** ... */ }

Notice the exports = main part. This is the root of the problem.

Consider a project that imports this library as a CJS module. Something along the lines of:

const merge = require('merge')

The compiler raises an error:

node_modules/merge/lib/src/index.js:4:17: ERROR - [JSC_UNDEFINED_VARIABLE] variable exports is undeclared
  4| module.exports = exports = main;

The compiler should not raise this error. The exports variable shortcut should be declared/well-known as per CJS standard in the same way module.exports is declared.

This issue is not only with the merge library. As far as I know, this library is generated with Webpack in a rather standard way. This means a great number of other libraries may be experiencing the same problem, whether they have been generated with Webpack or not. This means potentially a great number of libraries cannot be imported and used.

Note: Webpack is capable of importing the merge library without any problems.

Compiler Version: v20230103

Build command:

java -jar ./scripts/closureCompiler.jar \
  --entry_point=./src/js/index.js \
  --js=./src/**.js \
  --dependency_mode=PRUNE \
  --process_common_js_modules=true \
  --js=node_modules/merge/package.json \
  --js=node_modules/merge/**.js \
  --hide_warnings_for=node_modules \
  --warning_level=VERBOSE \
  --js_output_file=./dist/bundle.js \
  --module_resolution=NODE \
  --compilation_level=ADVANCED;