Problem

The cjsModuleRunnerPlugin was incorrectly transforming CommonJS require() calls by always accessing .default on the imported module:

// Before: Broken transformation
const x = (await import("some-package")).default;

This assumption fails when Vite's import analysis resolves the import to an ESM variant of a package. For example, when importing @swc/helpers/_/_interop_require_default, Vite may resolve it to @swc/helpers/esm/_interop_require_default.js. Pure ESM modules with only named exports don't have a .default property, causing .default to return undefined and breaking code that depends on these imports.

This issue was discovered when trying to load Next.js CommonJS modules that import from packages like @swc/helpers, resulting in errors like:

Cannot read properties of undefined (reading '_')

Solution

Introduced a runtime interop helper __cjs_interop__ that uses an explicit marker-based approach to distinguish transformed CJS modules from genuine ESM modules:

function __cjs_interop__(m) { 
  return m.__cjs_module_runner_transform ? m.default : m; 
}

All modules transformed by cjsModuleRunnerPlugin now export an explicit marker:

export const __cjs_module_runner_transform = true;

The helper only unwraps .default when the __cjs_module_runner_transform marker is present, ensuring we don't incorrectly unwrap .default on genuine ESM modules. This explicit approach is safer and more predictable than heuristic-based detection.

// After: Correct transformation
const x = __cjs_interop__(await import("some-package"));

Changes

• Updated transformCjsToEsm() to inject the interop helper and wrap all dynamic imports
• Added __cjs_module_runner_transform marker export to all transformed CJS modules
• Works for both top-level and hoisted require() calls
• Helper is only injected when require() calls are present in the code
• Updated test snapshots to reflect new transformation output

Testing

• ✅ All 6 unit tests pass with updated snapshots
• ✅ All 107 e2e tests pass
• ✅ Verified the fix handles CJS modules and doesn't incorrectly transform genuine ESM modules

Fixes #904

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