"TypeError: Cannot create proxy with a non-object as target or handler" when importing node module with a Proxy

[JoshuaKGoldberg]

Describe the bug

When importing from an installed npm package such as not-a-log that uses a Proxy, Vitest crashes with:

 FAIL  index.test.js [ index.test.js ]
TypeError: Cannot create proxy with a non-object as target or handler
 ❯ Object.get node_modules/not-a-log/not-a-log.js:9:12
 ❯ index.test.js:1:1
      1| import logger from "not-a-log";
       | ^
      2|

Reproduction

https://stackblitz.com/edit/vitest-dev-vitest-keoxtv

import logger from "not-a-log";

I also set up a full reproduction here: https://github.com/JoshuaKGoldberg/repros/tree/vitest-proxy-not-a-log

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 18.20.3 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.2.3 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    @vitest/ui: latest => 2.0.2 
    vite: latest => 5.3.3 
    vitest: latest => 2.0.2

Used Package Manager

npm

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
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• The provided reproduction is a minimal reproducible example of the bug.

[hi-ogawa]

Thanks for the reproduction. It looks like this is because "then" property access is send to not-a-log's proxy due to interop.

vitest/packages/vite-node/src/client.ts

Lines 547 to 553 in 5d6d801

	return new Proxy(mod, {
	get(mod, prop) {
	if (prop === 'default') {
	return defaultExport
	}
	return mod[prop] ?? defaultExport?.[prop]
	},

First of all, "then" access shouldn't happen for static import, but Vitest (Vite SSR) rewrite's all imports as dynamic import internally, so that part cannot be probably fixed (cf. #5122).

Technically, the same error can happen in innocent-looking async/await code outside of Vitest, so maybe not-a-log should be robust about this use case. For example, a following code causes same error on node:

// node repro.mjs

async function main() {
  // "await" implicitly checks `mod.default.then`
  await import("not-a-log").then(mod => mod.default);
}

main();

// or maybe this looks more innocent
async function main2() {
  const { default: logger } = await import("not-a-log");
  return logger;
}

// "await" implicitly checks `logger.then`
const logger = await main2();

One way to save error this on Vitest is to fix this TODO

vitest/packages/vite-node/src/client.ts

Lines 526 to 528 in 5d6d801

	// never interop ESM modules
	// TODO: should also skip for `.js` with `type="module"`
	return !path.endsWith('.mjs') && 'default' in mod

or you can try disabling this "interop" entirely by test.deps.interopDefault: false, which would preserve node import semantics.

export default defineConfig({
  test: {
    deps: {
      interopDefault: false,
    }
  },
})

[JoshuaKGoldberg]

Thanks for the deep dive & explanation! I filed an issue upstream: jimmywarting/not-a-log#2.

[sheremet-va]

I am not sure if this is a bug in Vitest. I don't think it's possible to support this case because then on an object is always called even if you return then from a module:

// mod.ts
export const then = () => {}

// index.ts
await import('./mod.ts')

[hi-ogawa]

Named export of then would be impossible (which is the case of actor library in #5122), but this issue is slightly different since Vitest is accessing then on default export object due to cjs interop.

Since not-a-log has type: "module", cjs interop should not be necessary, but Vitest does it currently due to this heuristics:

vitest/packages/vite-node/src/client.ts

Lines 526 to 528 in 5d6d801

	// never interop ESM modules
	// TODO: should also skip for `.js` with `type="module"`
	return !path.endsWith('.mjs') && 'default' in mod

actor library crashes on Vite SSR, but not-a-log works on Vite SSR.

[hi-ogawa]

I'm closing this for the same reason as #5122 and upstream vitejs/vite#18328