On-the-fly ESM babel transpilation

[make-github-pseudonymous-again]

I am experimenting with transforming a CommonJS module into an ECMAScript module. I use AVA to test this module. One thing I am struggling with is the following: when I set type to "module" in package.json the tests run fine but it seems it skips on-the-fly babel transpilation of sources (which I configured with the require config key). I understand transpilation is unnecessary for the tests to run correctly, however, part of babel's job is to remove debug statements that should only be displayed when running with NODE_ENV=debug or BABEL_ENV=debug. In the current state, those debug statements are displayed regardless of the value of those environment variables.

How should I configure AVA so that it strips debug statements when I run my tests outside of the debug environment?

[novemberborn]

ESM uses a different loading mechanism. Your on-the-fly transpiler needs to hook into this. It's nothing to do with AVA but is available as an experimental Node.js feature instead. You should be able to find out more by asking the transpiler project.

[make-github-pseudonymous-again]

OK I believe I found the culprit. In AVA v3.15 ESM support is probed with

ava/lib/worker/subprocess.js

Lines 9 to 16 in 32c5425

	const supportsESM = async () => {
	try {
	await import('data:text/javascript,'); // eslint-disable-line node/no-unsupported-features/es-syntax
	return true;
	} catch {}
	return false;
	};

Node 16.2.0 runs just fine:

$ node
Welcome to Node.js v16.2.0.
Type ".help" for more information.
> import('data:text/javascript,').then(x => console.log({x})).catch(err => console.error({err}))
Promise { <pending> }
> { x: [Module: null prototype] {  } }

but with babel it breaks

$ node  --loader=@node-loader/babel
(node:2643346) ExperimentalWarning: --experimental-loader is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
Welcome to Node.js v16.2.0.
Type ".help" for more information.
> import('data:text/javascript,').then(x => console.log({x})).catch(err => console.error({err}))
Promise { <pending> }
> {
  err: TypeError [ERR_INVALID_URL_SCHEME]: The URL must be of scheme file
      at new NodeError (node:internal/errors:363:5)
      at Object.fileURLToPath (node:internal/url:1374:11)
      at transformSource (file:///.../node_modules/@node-loader/babel/lib/node-loader-babel.js:11:19)
      at Loader.moduleStrategy (node:internal/modules/esm/translators:142:28)
      at processTicksAndRejections (node:internal/process/task_queues:96:5)
      at async link (node:internal/modules/esm/module_job:64:21) {
    code: 'ERR_INVALID_URL_SCHEME'
  }
}

In AVA v3.15 this is caught then handled here

ava/lib/worker/subprocess.js

Line 201 in 32c5425

ipc.send({type: 'internal-error', err: serializeError('Internal runner error', false, new Error('ECMAScript Modules are not supported in this Node.js version.'))});

[make-github-pseudonymous-again]

The support probe above was introduced in #2382 then #2404. Then ava/lib/worker/subprocess.js simply disappears. Currently hunting for the equivalent in AVA v4.

[make-github-pseudonymous-again]

OK. It moves to ava/lib/worker/base.js in 0e5cc7d#diff-1fafe951c8b8236f0451cecb2a2fa333ab8dcf74f89999c87b21d76b2bde3ed8.

[make-github-pseudonymous-again]

Support detection is removed before that in a7faa3e#diff-80848ce32228881bbdf2750d3761102e26a1247e1937bf7330714874e0bd8f35

[make-github-pseudonymous-again]

@novemberborn So I guess the conclusion is that Babel v7.14.x which I am running does not support data: urls when transpiling dynamic import calls. But since support detection was removed it now works correctly with AVA v4, without fixing Babel.

[make-github-pseudonymous-again]

After some research I came up with the following solution:

• Use AVA v4 (because AVA v3.15 will not work after being transpiled by Babel).
• Add devDependency @node-loader/babel
• Add "--experimental-loader=@node-loader/babel" to "nodeArguments" key in AVA's config
• Configure babel to produce ESM output for the test, cover, and debug environments.

Example diff edit-distance/myers-1986@8c1b1cb

[novemberborn]

AVA 4 targets Node.js versions that support ESM, so there's no more need for feature detection.