CJS support

[make-github-pseudonymous-again]

I found at least one offensive line here:

node-loader-babel/lib/node-loader-babel.js

Lines 46 to 48 in 4d53080

	// Maybe a shaky assumption
	// TODO: look at babel config to see whether it will output ESM/CJS or other formats
	format: "module",

If I replace the format definition with format: /\.cjs$/.test(filename) ? "commonjs" : "module", it works for me so I assume something was overlooked here. This may not be the best way to handle this but at least it shows I can fix it to work for my use case.

If I do not do that, I get tons of errors similar to SyntaxError: The requested module '#foo' does not provide an export named bar when importing some commonjs module. I am also using @node-loader/import-maps so maybe there is some interdependence there?

[make-github-pseudonymous-again]

I found a better way is to rely on the original format through defaultLoad, for exampe:

const {format, source} = await defaultLoad(url, context, defaultLoad);

This solves my use-case but is still not a solution to getting the output format through babel. Although I am not sure it makes sense for babel to transform from ESM to CJS in this particular case because the load hook cannot emit source for the commonjs format (see https://nodejs.org/api/esm.html#loadurl-context-defaultload).

[jolyndenning]

If I do not do that, I get tons of errors similar to SyntaxError: The requested module '#foo' does not provide an export named bar when importing some commonjs module. I am also using @node-loader/import-maps so maybe there is some interdependence there?

This is expected, since commonjs modules do not provide named exports. Recently, Guy Bedford added a lexer to node that attempts to provide named exports for commonjs modules, but this is not guaranteed to work in all cases. See https://nodejs.org/dist/latest-v17.x/docs/api/esm.html#import-statements, which says "When importing CommonJS modules, the module.exports object is provided as the default export. Named exports may be available, provided by static analysis as a convenience for better ecosystem compatibility."

Is the problem you're encountering that the static analysis of that lexer is skipped when a loader is applied? If so, I think that the best way to solve that would be to allow the user of node-loader-babel to customize the useLoader function.

[make-github-pseudonymous-again]

OK. I thought I had pinpointed it. See explanation in #8 (comment). I will double-check and come back to you.

PS: BTW @joeldenning, thanks for your great work on this software. Other than this minor "issue", it has worked wonders so far.

[make-github-pseudonymous-again]

I created a reproduction repository at https://github.com/bisectifiate/node-loader-babel-commonjs.

I tested the following (:heavy_check_mark: means it works, :heavy_multiplication_x: means it does not work):

• Importing named commonjs exports from module:
  • without loader ✔️
  • with @node-loader/babel ✖️
  • with a custom babel loader ✔️
• Importing named module exports from module:
  • without loader ✔️
  • with @node-loader/babel ✔️
  • with a custom babel loader ✔️
• Importing default module exports from module:
  • without loader ✔️
  • with @node-loader/babel ✔️
  • with a custom babel loader ✔️

It seems commonjs named exports work without a loader or with a patched babel loader.

To rule out static analysis as the culprit, there is this PR which imports the commonjs source with a default import (import x from) rather than a named import (import {y} from). @node-loader/babel still fails in this case even though we do not use named exports requiring static analysis. The error is:

The requested module '../../src/commonjs-named.cjs' does not provide an export named 'default'

Note that this use case works without a loader or with a patched babel loader.

What I think is happening with @node-loader/babel is that a commonjs source is passed to babel as null, babel outputs null, and @node-loader/babel wraps this null as {format: 'module', source: null} which creates an empty module, with no named or default exports. If that is indeed the case, what @node-loader/babel should be doing, and that is what the patched version and #8 do, is to output {format: 'commonjs', source: null} when given an input with format commonjs (and hence with source null). See the table at https://nodejs.org/api/esm.html#loadurl-context-defaultload that says source is not applicable when format is 'commonjs':

The value of source is ignored for type 'commonjs' because the CommonJS module loader does not provide a mechanism for the ES module loader to override the CommonJS module return value. This limitation might be overcome in the future.

Just for your information, importing the default export from a commonjs input (exports = foobar) does not work at all (it does not work in any of the configurations: loader or no loader). See this PR. At the moment, I am not interested in making this kind of default export work. EDIT: OK, it does work if you use module.exports = foobar.

[make-github-pseudonymous-again]

What I think is happening with @node-loader/babel is that a commonjs source is passed to babel as null, babel outputs null, and @node-loader/babel wraps this null as {format: 'module', source: null} which creates an empty module, with no named or default exports.

I was able to replicate @node-loader/babel's behavior with a minimal amount of code here. What happens is that babel transpiles null to the empty string ''. Transpiling to null would yield a more direct error. The conclusion stays the same however.

[jolyndenning]

Fix released in https://github.com/node-loader/node-loader-babel/releases/tag/v2.0.1

[jolyndenning]

Added a bit of documentation that's available at https://github.com/node-loader/node-loader-babel#commonjs-modules