Support native ESM configs in Node 13, support untranspiled configs

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

resolves #3443
resolves #2953

This will add support for native ESM and untranspiled configs, which will solve two issues:

• If a config is in a "type": "module" folder, it could not import real ESM plugins in Node 13
• if a config imports a plugin from a nested folder, the __dirname and import.meta.url were wrong

Support will be based on file extension:

• If the extension is .cjs, the config will assumed to be CommonJS and will be required untranspiled
• If the extension is .mjs AND the Node version is at least 13, the config will be assumed to be native ESM and will be imported untranspiled
• Otherwise the old logic (transpiling to CJS) will be used

TODO:

• Add helpful error messages for common issues that point users to using the correct extensions and file format
• Automatically check for configs in the following order: rollup.config.mjs, rollup.config.cjs, rollup.config.js.

[rollup-bot]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#load-untranspiled-config-file

or load it into the REPL:
https://rollupjs.org/repl/?circleci=10188

[codecov]

Codecov Report

Merging #3445 into master will increase coverage by 0.72%.
The diff coverage is 94.41%.

@@            Coverage Diff             @@
##           master    #3445      +/-   ##
==========================================
+ Coverage   95.08%   95.81%   +0.72%     
==========================================
  Files         171      174       +3     
  Lines        5861     5898      +37     
  Branches     1732     1737       +5     
==========================================
+ Hits         5573     5651      +78     
+ Misses        156      127      -29     
+ Partials      132      120      -12     

Impacted Files	Coverage Δ
cli/run/build.ts	89.47% <88.88%> (+9.47%)	⬆️
cli/run/getConfigPath.ts	89.47% <89.47%> (ø)
cli/run/watch.ts	84.93% <93.02%> (+40.93%)	⬆️
cli/run/loadConfigFile.ts	95.55% <95.34%> (+11.55%)	⬆️
cli/run/commandPlugins.ts	96.87% <96.87%> (ø)
cli/run/index.ts	100.00% <100.00%> (+3.52%)	⬆️
cli/run/loadConfigFromCommand.ts	100.00% <100.00%> (ø)
cli/run/resetScreen.ts	100.00% <100.00%> (+37.50%)	⬆️
src/utils/mergeOptions.ts	100.00% <100.00%> (ø)
src/watch/watch.ts	99.00% <100.00%> (+0.96%)	⬆️
... and 6 more

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[brettz9]

With the latest Rollup release, despite this merge, with /Users/brett/rollup-plugin-filesize/rollup.config.js:

import filesize from "./dist/index-esm.js";

...and /Users/brett/rollup-plugin-filesize/dist/index-esm.js:

console.log(__dirname);

Expected

/Users/brett/rollup-plugin-filesize/dist/

Actual

/Users/brett/rollup-plugin-filesize/

[lukastaegert]

Please read the docs, you will only get an untranspiled config if you rename your config with an .mjs in Node 13 or .a cjs extension. Also I am not sure you can use __dirname in a native Node ES module. https://rollupjs.org/guide/en/#using-untranspiled-config-files

[brettz9]

Ah, my apologies, thank you!

[brettz9]

... you will only get an untranspiled config if you rename your config with an .mjs in Node 13 or .a cjs extension

and from the docs:

By default, Rollup will expect config files to be ES modules and bundle and transpile them and their relative imports to CommonJS before requiring them.

But in my example, my relative imports are ESM, so shouldn't that work? (And I am on Node 10, FWIW.) I'm sorry to belabor this if I'm the one missing something, but I had taken a look at the docs, but was just unclear (and still am).

Re: __dirname, yes, you are right, it is not to be supported (per https://nodejs.org/dist/latest-v12.x/docs/api/esm.html#esm_no_require_exports_module_exports_filename_dirname ), but I get the same results with import.meta.url.

[lukastaegert]

and bundle and transpile them

This is what breaks import.meta.url and __dirname. Maybe this side-effect should be stated explicitly. A bundle of course has the __dirname of the bundled file. If you are stuck on Node 10, your only option for correct __dirname in relative imports is to switch to .cjs.

[brettz9]

I would understand the bundle having the __dirname or import.meta.url of the bundled file (being within dist where it is output). But these end up instead being the Rollup config file path--i.e., /Users/brett/rollup-plugin-filesize/rollup.config.js is what import.meta.url gives.

And even when testing on Node 12 with the rollup.config.mjs file, I am getting the same result--import.meta.url is showing the path of the rollup.config.mjs config file--not the path of the source file in which the logging statement is present (nor of the destination file).

[brettz9]

FWIW, with rollup.config.cjs (and its change to point to the dist file, built with @babel/plugin-syntax-import-meta), things are working as expected for me, however.

[lukastaegert]

And mjs is only working natively in Node 13, as stated in the docs. Otherwise the config file is the bundled file, hence everything points there.

[brettz9]

Ah, ok, I see. Re: the config file itself being the bundled file, I see now how that can make sense.

Re: Node 13, my apologies, and thank you very much for your patience and help.