Feature Request: support mjs config

[xiaoxiangmoe]

We need support .postcssrc.mjs or postcss.config.mjs.
Also we need support .postcssrc.cjs or postcss.config.cjs.

[ai]

Sure. Send PR.

[xiaoxiangmoe]

I found loading esm module is async, it seems it will break sync load config.

https://github.com/postcss/postcss-load-config#sync

Maybe we can only use mjs in async mode. Is this work as intended?

[ai]

We can add .cjs support

[xiaoxiangmoe]

What should we do about mjs support?😂

[ai]

What should we do about mjs support?

Sorry, have no idea

[michael42]

Getting the basic support working shouldn't be hard. With lilconfig@^2.0.5, the following patch mostly works:

diff --git a/src/index.js b/src/index.js
index db5c40e..56a4538 100644
--- a/src/index.js
+++ b/src/index.js
@@ -81,14 +81,19 @@ const addTypeScriptLoader = (options = {}, loader) => {
       `.${moduleName}rc.ts`,
       `.${moduleName}rc.js`,
       `.${moduleName}rc.cjs`,
+      `.${moduleName}rc.mjs`,
       `${moduleName}.config.ts`,
       `${moduleName}.config.js`,
-      `${moduleName}.config.cjs`
+      `${moduleName}.config.cjs`,
+      `${moduleName}.config.mjs`
     ],
     loaders: {
       ...options.loaders,
       '.yaml': (filepath, content) => yaml.parse(content),
       '.yml': (filepath, content) => yaml.parse(content),
+      '.js': (filepath) => import(filepath).then(module => module.default),
+      '.cjs': (filepath) => import(filepath).then(module => module.default),
+      '.mjs': (filepath) => import(filepath).then(module => module.default),
       '.ts': loader
     }
   }

But the details are tricky:

• I also switched .js to support ECMAScript modues (CommonJS is also supported by import(...)) for projects using "type": "module" in the package.json
• import(...) requires running jest with NODE_OPTIONS=--experimental-vm-modules
• import(...) requires at least Node 12, but postcss-load-config still supports Node 10
• import(...) does not support synchronously loading config files at all

[ai]

Hm. We could think about major release. When is Node 12 end of support?

[michael42]

Node 12 seems to have 2022-04-30 as end of life and Node 10 went EOL on 2021-04-30.

[ai]

Is it possible to check import support in ".js" block and use require for old Node.js versions?

[michael42]

Probably, but it's a new syntax, not just a function, so it requires some sort of eval-based check and would complicate the tests even more. But, on the other hand, it could then be used in the synchronous API, too (albeit only the async API would actually support .mjs and ESM in .js).

[xiaoxiangmoe]

Is it possible to check import support in ".js" block and use require for old Node.js versions?

Maybe we can check node version

[ai]

Can we make PR to play with this approach?

[michael42]

I created #234, implementing require with a fallback to import(...). I noticed that (in contrast to my earlier comment import can be easily checked with try-catch block because Node 10 already parses import(...), it just always throws an error (when executing). Only Node 8 does not even parse it. But since that won't work when trying to load a config file in sync mode, I switched it around to prefer require and only use import(...) when necessary.

And just a warning: Node often crashes with a Segmentation fault when running the jest tests in that PR. Not sure why, though, maybe jest exposes a Node/V8 Bug?

Stack trace of thread 47614:
 #0  0x000055aa9c2cbaf0 n/a (node + 0x626af0)
 #1  0x000055aa9c75b7a7 _ZN2v88internal7Isolate38RunHostImportModuleDynamicallyCallbackENS0_6HandleINS0_6ScriptEEENS2_INS0_6ObjectEEENS0_11M>
 #2  0x000055aa9cae66cf _ZN2v88internal25Runtime_DynamicImportCallEiPmPNS0_7IsolateE (node + 0xe416cf)

[ai]

Very strange. On what Node.js versions?