@rollup/plugin-commonjs: circular require receives undefined instead of {} before module.exports =

[trackoor]

In @rollup/plugin-commonjs 29.0.3 (with rollup 4.68.0), a module with a top-level module.exports = ... assignment and no other module or exports references leaves its exports variable uninitialized (var a;) until the assignment executes. If another module requires it back in a cycle before that assignment runs, require() returns undefined instead of the initial {} exports object.

// a.js
const b = require('./b.js');
module.exports = { seenByB: b.seen };

// b.js
const a = require('./a.js');
module.exports = { seen: typeof a };

// entry.js
console.log(require('./a.js'));

Bundling entry.js with @rollup/plugin-commonjs (default strictRequires: true or 'auto') and running the output prints { seenByB: 'undefined' }. Running node entry.js directly prints { seenByB: 'object' }, as required by Node's CommonJS cycle semantics (an unfinished module returns its initial {} exports object).

The same uninitialized var also causes top-level this accesses before module.exports = ... (e.g. this.x = 1; module.exports = { y: 2 };) to throw TypeError: Cannot set properties of undefined.

Credit: https://ucsbplse.github.io/

[ItsRenjiii]

Confirmed with rollup 4.68.0 and @rollup/plugin-commonjs 29.0.3. The bundle prints { seenByB: 'undefined' } for every strictRequires setting (true, 'auto', 'debug', false), and the this.x = 1 case throws.

The cause is in transform-commonjs.js. A module whose only export is one top-level module.exports = ... is handled in replace mode, and when it gets a require wrapper its exports variable is declared as var a;, with no value. Node starts every module with exports = {}, so a circular require or a top-level this that runs before the assignment sees {} in Node and undefined in the bundle.

I've opened a PR with the fix: #2045. It sets a = {}; when the first require starts and, for unwrapped modules that use top-level this, declares var a = {}; before the body. It adds your two cases as tests, including the this case under strictRequires: 'auto' and false. They fail on master and pass with the change, and the plugin's full suite (296 tests) passes on Node 20 and 24.

Until that's released, either of these avoids replace mode and gives the Node result:

// keep the module.exports reference, without replacing the object
Object.assign(module.exports, { seenByB: b.seen });

// or assign properties
exports.seenByB = b.seen;

Both print { seenByB: 'object' } on 29.0.3.