I've been exploring a fix for this issue, which checks if the environment supports dynamic imports and uses require otherwise, by using simple if checks to determine if Jest is running.

As mentioned in the PR description, the Jest VM doesn't allow dynamic imports by default; to make it work, Jest needs to run with a --experimental-vm-modules flag.

Dynamic imports work in both ESM and CommonJS, so from what I can see, this is a Jest-specific issue.

I don't think that we should address it in PHP-wasm by switching between import and require, and see two potential paths forward.

• We could accept it as is and document that Jest will require the --experimental-vm-modules flat to work with PHP-wasm.
• We could build a .cjs version of the PHP-wasm JS file, but these files are already 900+ KB, and adding separate cjs files would significantly increase the package size.

Looking at both options, my suggestion is to require the --experimental-vm-modules flag for Jest to work.

Is there a third option that I'm missing? Are there other places besides Jest where dynamic loading is an issue?

@adamziel @brandonpayton @mho22, what do you think?

@bgrgicak I fully agree with you.

I think we have another complex option that could make this work. Print separate banners in CJS and ESM during build. This works with ESBuild or Vite. A Vite plugin could be created to add banners for tests.

The last missing piece would be running the different packages locally, probably inside node-es-module-loader/loader.mts and we will maybe have a node-cjs-loader/loaders.cts that would add that part too for CJS mode ?

However, this is a lot more complicated process compared to adding the --experimental-vm-modules flag in Jest.

If you want to test your local changes with this test, you can use this process:

In one tab run nvm use 23; npm run local-package-repository.

In another cd packages/playground/test-built-npm-packages/commonjs-and-jest
Open package.json and replace the @wp-playground/cli with a URL to that package on the local package repository.
Install the package with npm install.
Run test with npm run test .

This error seems to be highly specific to jest:

cloudnik@MacBook-Pro-4 ~/w/A/c/p/p/testcjs (trunk) [1]> npx jest ./                                                                                                                                          (base) 

 RUNS  ./test.spec.js
node:internal/modules/esm/utils:271
    throw new ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING_FLAG();
          ^

TypeError: A dynamic import callback was invoked without --experimental-vm-modules

(test.spec.js)

it('should load the PHP loader module', () => {
	require('../dist/packages/php-wasm/node-builds/8-3/index.cjs').getPHPLoaderModule().then(console.log, console.error)
})

I couldn't, for the life of me, reproduce that error with vanilla Node 22+ without jest involved. I've tried:

> node                                                                                     (base) 
Welcome to Node.js v22.21.0.
Type ".help" for more information.
> require('./dist/packages/php-wasm/node-builds/8-3/asyncify/php_8_3.js');
[Module: null prototype] {
  dependenciesTotalSize: 27938593,
  dependencyFilename: '/Users/cloudnik/www/Automattic/core/plugins/playground/dist/packages/php-wasm/node-builds/8-3/asyncify/8_3_28/php_8_3.wasm',
  init: [Function: init]
}

It doesn’t break with node test.cjs with package.json where "type": "commonjs" is set and that has the following content

require('../dist/packages/php-wasm/node-builds/8-3/index.cjs').getPHPLoaderModule().then(console.log, console.error)

This also worked (although with some warnings):

esbuild --bundle --target=node22 --platform=node ./test.cjs

npx ts-node also worked.

What are you doing differently, jest?

jest seems to be using the node:vm module to run the tested code, and Node.js seems to be much more limited around ESM interop in that context:

Thank you for opening this PR and exploring the space of possibility!

CJS and ESM interop is really difficult to get right, and Jest's custom runtime makes it even more difficult. Given the amount of headache the current CJS+ESM plumbing is already giving us, I'd rather not make it even more complex. Instead, let's just officially make jest unsupported without the --experimental-vm-modules. I've opened #3121 to at least provide clear error messages in typical jest failure modes.

Thanks @bgrgicak and @adamziel for taking look at this.