Reproduction link or steps

This came from investigating vitejs/vite#22435, but the behavior reproduces with Rolldown directly and without Backbone.

A minimal fixture added under crates/rolldown/tests/rolldown/function/experimental/strict_execution_order/cjs_namespace_late_mutation:

// main.js
import * as target from './target.cjs';
import './augment.cjs';

export const result = {
  existing: target.existing,
  addedType: typeof target.added,
  defaultAddedType: typeof target.default?.added,
};

// target.cjs
module.exports = {
  existing: 'existing',
};

// augment.cjs
const target = require('./target.cjs');

target.added = function added() {
  return 'added';
};

// _test.mjs
import assert from 'node:assert';

const { result } = await import('./dist/main.js');

assert.deepStrictEqual(result, {
  existing: 'existing',
  addedType: 'function',
  defaultAddedType: 'function',
});

// _config.json
{
  config: {
    input: [
      {
        name: main,
        import: ./main.js
      }
    ]
  },
  expectExecuted: false,
  snapshot: false
}

Run:

NEEDS_EXTENDED=false cargo run-fixture crates/rolldown/tests/rolldown/function/experimental/strict_execution_order/cjs_namespace_late_mutation/_config.json

What is expected?

The namespace object should observe target.added after the side-effect CommonJS module has executed, matching ESM static import execution semantics and the behavior of Rollup + @rollup/plugin-commonjs for the same fixture.

In other words, result should be:

{
  existing: 'existing',
  addedType: 'function',
  defaultAddedType: 'function',
}

What is actually happening?

The test fails with:

AssertionError [ERR_ASSERTION]: Expected values to be strictly deep-equal:
+ actual - expected

  {
+   addedType: 'undefined',
-   addedType: 'function',
    defaultAddedType: 'function',
    existing: 'existing'
  }

The generated output initializes the CJS namespace wrapper before the later side-effect require mutates the same CommonJS export object:

var import_target = __toESM(require_target(), 1);
require_augment();

__toESM snapshots/copies the current own properties from module.exports, so target.added is missing on the namespace object. target.default.added works because default still points to the mutable original CommonJS export object.

A named import variant works because the output keeps a direct reference to the CJS export object and reads the property after the side-effect module runs:

var import_target = require_target();
require_augment();
typeof import_target.added;

System Info

OS: Windows 11
Node: v24.14.0
pnpm: 11.1.2
rustc: 1.95.0 (59807616e 2026-04-14)
cargo: 1.95.0 (f2d3ce0bd 2026-03-21)
Rolldown repo commit tested: bf468a4dc

Any additional comments?

This affects Vite 8 production builds for legacy CommonJS packages that augment another CommonJS package via side effects. One concrete downstream case is backbone-relational augmenting backbone: import * as Backbone from backbone; import backbone-relational works in Vite dev and Vite 7 build, but fails in Vite 8 build because the namespace wrapper is created before the augmentation runs.