ESM Package Testing

Issue

We have a package dependency with both package.json:exports and a "type": "module" declaration. The issue is -- can CJS work at all?

Conclusion

There is no way to require files from a package.json that has "type": "module" declared, even if it has package.json:exports.*.require entries.

Usage

ESM

Try the ESM version first and verify it works...

$ node index.mjs
{ msg: 'ESM' }

CJS

Now try the CJS version and get a failure:

$ node index.js
# Expected: { msg: 'CJS' }
/Users/rye/scm/fmd/esm-test-pkg/index.js:2
const msg = require("my-pkg");
            ^

Error [ERR_REQUIRE_ESM]: require() of ES Module /Users/rye/scm/fmd/esm-test-pkg/node_modules/my-pkg/cjs.js from /Users/rye/scm/fmd/esm-test-pkg/index.js not supported.
cjs.js is treated as an ES module file as it is a .js file whose nearest parent package.json contains "type": "module" which declares all .js files in that package scope as ES modules.
Instead rename cjs.js to end in .cjs, change the requiring code to use dynamic import() which is available in all CommonJS modules, or change "type": "module" to "type": "commonjs" in /Users/rye/scm/fmd/esm-test-pkg/node_modules/my-pkg/package.json to treat all .js files as CommonJS (using .mjs for all ES modules instead).

    at Object.<anonymous> (/Users/rye/scm/fmd/esm-test-pkg/index.js:2:13) {
  code: 'ERR_REQUIRE_ESM'
}

Node.js v18.7.0

Notes

Changing CJS suffix would work

Changing node_modules/my-pkg/cjs.js to node_modules/my-pkg/cjs.cjs with the .cjs suffix, would cause the program to work:

diff --git a/node_modules/my-pkg/cjs.js b/node_modules/my-pkg/cjs.cjs
similarity index 100%
rename from node_modules/my-pkg/cjs.js
rename to node_modules/my-pkg/cjs.cjs
diff --git a/node_modules/my-pkg/package.json b/node_modules/my-pkg/package.json
index bb7f270..af7610e 100644
--- a/node_modules/my-pkg/package.json
+++ b/node_modules/my-pkg/package.json
@@ -9,7 +9,7 @@
   "exports": {
     ".": {
       "import": "./esm.js",
-      "require": "./cjs.js"
+      "require": "./cjs.cjs"
     }
   }
 }