ERR_REQUIRE_ESM with import of `*.js` from `ley.config.mjs`

[karlhorky]

Hi @lukeed 👋 hope things are going well!

Encountered an unusual problem the other day:

1. CommonJS project (no "type": "module" in package.json)
2. ley.config.mjs file which imports a .js file, which also uses ESM
3. ERR_REQUIRE_ESM error, trying to require() the ley.config.mjs file 🤔

CodeSandbox: https://codesandbox.io/p/sandbox/lucid-babycat-74sz59?file=%2Fley.config.mjs%3A3%2C19

$ ley up
node:internal/errors:490
    ErrorCaptureStackTrace(err);
    ^

Error [ERR_REQUIRE_ESM]: require() of ES Module /workspace/ley.config.mjs not supported.
Instead change the require of /workspace/ley.config.mjs to a dynamic import() which is available in all CommonJS modules.
    at exports.load (/workspace/node_modules/ley/lib/util.js:62:10)
    at async /workspace/node_modules/ley/bin.js:11:13 {
  code: 'ERR_REQUIRE_ESM'
}

Node.js v18.16.0
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

I guess I would instead expect the error that Node.js throws here, complaining that A) it cannot find the named export and B) that ley-import-broken.js is a CommonJS module:

$ node ley.config.mjs 
file:///Users/k/p/a/ley.config.mjs:1
import { a } from './ley-import-broken.js';
         ^
SyntaxError: Named export 'a' not found. The requested module './ley-import-broken.js' is a CommonJS module, which may not support all module.exports as named exports.
CommonJS modules can always be imported via the default export, for example using:

import pkg from './ley-import-broken.js';
const { a } = pkg;

    at ModuleJob._instantiate (node:internal/modules/esm/module_job:124:21)
    at async ModuleJob.run (node:internal/modules/esm/module_job:190:5)

Node.js v18.17.0

Somewhat related to this other issue by @mplibunao:

• https://github.com/lukeed/ley/issues/27

[karlhorky]

Ley's migrate up is however still failing, so having a different error message is not as much a real problem as it is confusing...

[karlhorky]

Ideally what I would love would be to have ley.config.js + all .js imports treated as ESM, even without "type": "module" in package.json (or maybe ley.config.ts supported, when tsm is used - which would also unlock ESM syntax)

But yeah, can also see downsides here...

[lukeed]

You can't do this for Node reasons, not ley. You can only (technically) have ESM within .js with "type":"module" in your pkg config. Thats how native loaders can know what to expect/allow. (Bundlers can be different story.. aka more permissive.) This was done because ESM was a later, additive feature, and .js had to remain IIFE/CommonJS-oriented by default... type: module turns on the ESM assumption.

The reason you're seeing ERR_REQUIRE_ESM is because import() is tried first, and falls back to require. Node is throwing an error (with a stderr warning, see CSB) and so the require() tried again, also throwing an error.

[karlhorky]

Ok good, so it sounds like this is "by design" and Ley won't go in the direction of bundler behavior? If so, we can probably close this issue and leave it as being documented.

[lukeed]

It's just plain Node loading / ESM semantics. I don't think it needs documenting since it applies to every module (and .js file) within the ecosystem. There isn't anything ley specific about the situation.

thanks~!