Weird Behavior When Mixing CommonJS & ESM Imports

[djdjz7]

I am trying to use MathJax with bun when I came across some very weird bug which prevents MathJax from being imported correctly.

This does not appear with Node.js (v25.2.1), but does with Bun (1.3.3).

Reproduction

I managed to reproduce the same problem with just three files:

• entry.js

  import './setup.js';
  import './test.js';

• setup.js

  global.Foo = { bar: 42 };

• test.js

  exports.foo;
  console.log(global.Foo.bar);

then run

bun entry.js    # or node entry.js

bun will complain about TypeError:

2 | exports.foo;
                       ^
TypeError: undefined is not an object (evaluating 'global.Foo.bar')
      at <anonymous> (/bun-import-test/test.js:2:20)

as if setup.js has never been evaluated...

while node will output 42 just fine.

Notes

It's worth mentioning that it seems the exports.foo line in test.js is causing the issue. Remove the line, and everything will be fine.

[djdjz7]

Some updates:

if we change entry.js to

import './setup.js';
await import('./test.js');

Bun will also print 42 🤔

So is this related to async module resolution? Maybe I should file an issue on the MathJax side?

[RiskyMH]

Yeah this does look like very risky code because imports are implicitly async so they shouldn't really be relying on sideffects like that imo