Make native Node ESM support configurable

[tbranyen]

Is your feature request related to a problem or a nice-to-have?? Please describe.

Relates to: #4038

Currently unable to use "type": "module" in my mono-repo package as then Mocha runs all tests assuming native node, instead of using the configured @babel/register extensions loader. There seems to be a conflation that because a package is designated as native Node ESM, that means the tests should run that way. There is a big distinction in my case.

I import my code using the extension-less form:

import { innerHTML } from './inner-html';

(See build failure in Additional context below)

Describe the solution you'd like

I would like to see an option to be able to disable Mocha's Node interpretation, regardless of .mjs or "type": "module" in the root package.json.

Something like:

--disable-esm

Describe alternatives you've considered

• Disabling the feature to import Node ESM from the root of my project
• Converting all my imports to have an extension

Additional context

λ NODE_ENV=test+cov nyc -r=lcov mocha test/_setup test test/integration/*.js


Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/home/tim/git/diffhtml/packages/diffhtml/lib/util/types' imported from /home/tim/git/diffhtml/packages/diffhtml/lib/util/pool.js
    at finalizeResolution (internal/modules/esm/resolve.js:285:11)
    at moduleResolve (internal/modules/esm/resolve.js:708:10)
    at Loader.defaultResolve [as _resolve] (internal/modules/esm/resolve.js:798:11)
    at Loader.resolve (internal/modules/esm/loader.js:100:40)
    at Loader.getModuleJob (internal/modules/esm/loader.js:246:28)
    at ModuleWrap.<anonymous> (internal/modules/esm/module_job.js:47:40)
    at link (internal/modules/esm/module_job.js:46:36)

[juergba]

@tbranyen I haven't really understood your request, but it seems very weird to me to configure a project as ESM (via .mjs or type: module) and then instruct Mocha to do the opposite.

Have you noted this in Babel's docs?

Note: @babel/register does not support compiling native Node.js ES modules on the fly, since currently there is no stable API for intercepting ES modules loading.

Have you tried this: babel-register-esm?

@giltayar cc

[tbranyen]

@juergba ah I missed that in the babel register docs 👍. This may be a non-issue for Mocha in that case.

[giltayar]

If you'd like to disable ESM, just don't put "type": "module" in the package.json, and all will be well. Putting that in the package.json signals to Node.js that you want all .js files to be ESM native.

But the ecosystem is slowly but inexorably moving to native ESM, so I would definitely investigate moving to ESM. If you need on the fly babel transpilation, like @babel/register does, then use the babel-register-esm plugin, as @juergba suggested:

npm install babel-register-esm
mocha --loader babel-register-esm testfile.js

[tbranyen]

@giltayar @juergba I've been able to confirm my problem using plain Node and @babel/register as well (no mocha). Shame, because all my other tooling still works since they don't "shadow" the node process like Mocha does. I use the "type": "module" attribute value since I have some files in the root that re-export from a safe Node ESM directory:

export * from './dist/es/lite.js';

The rest of the project is not necessarily "Node ESM". Given that this is not a uniquely Mocha problem, I think it's safe to close. I will investigate using a Node ESM loader to allow extension-less imports as that seems to be the only real blocker ATM.

[tbranyen]

Looks like Node provides an option to support extension-less imports via https://nodejs.org/api/esm.html#customizing-esm-specifier-resolution-algorithm: --experimental-specifier-resolution=node.

I set the following config in .mocharc.json, but haven't had success yet. Will update if I can get it working as this would completely solve the original ask.

{
  "node-option": [
    "experimental-specifier-resolution=node"
  ]
}