CJS / ES Module?

[jycouet]

I'm using reflect-metadata in a SvelteKit project, and it's working great 👍

I have a warning at the start:
reflect-metadata doesn't appear to be written in CJS, but also doesn't appear to be a valid ES module (i.e. it doesn't have "type": "module" or an .mjs extension for the entry point). Please contact the package author to fix.

Maybe this message speaks better to you?
Is it the way you distribute the package?

[geongeorge]

Same error for me when trying to add nest js to a vite project

[dstampher]

I am new to TS. I am using typeorm, which requires this library. I cannot set tsconfig module type to esnext, it must be commonjs when I import "reflect-metadata". And that is causing other problems in my application that are very annoying for a newbie.

[patrickjm]

I got this to work importing from a package with "type": "module" and using yarn v3 / berry:

Create this patch file:

diff --git a/package.json b/package.json
index 11d979c..e07509d 100644
--- a/package.json
+++ b/package.json
@@ -4,6 +4,7 @@
   "description": "Polyfill for Metadata Reflection API",
   "main": "Reflect.js",
   "types": "index.d.ts",
+  "type": "module",
   "typescript": {
     "definition": "Reflect.d.ts"
   },

Then in your package json, add this resolutions block:

"resolutions": {
    "reflect-metadata": "patch:[email protected]#./path/to/your/patchfile.patch"
  },

If you are using a monorepo, this has to be at the top level package.json regardless of where you are importing reflect-metadata.

[ljharb]

Nothing should ever need type:module; if it's ESM it'd have .mjs, so I'm not sure why vite is failing to properly recognize https://unpkg.com/browse/[email protected]/Reflect.js as CJS.

It doesn't export anything, since it just mutates the global Reflect, but since that's not a requirement of CJS, this seems like a vite bug, plain and simple.

[jycouet]

You know probably way better than me the cjs / mjs issue in general.

If I understand it correctly:

• if your file is .cjs everyone knows that it's a commonjs file.
• if your file is .mjs everyone knows that it's a esm file.

Now, if you have a .js no one knows... it depends on your package.json to select what is your "default" way of doing.

More and more lib ship in both mode: esm & cjs & a default with these kind of exports:

Like this, tooling can pickup the right things for them.

[AirMonkeys-Kun]

reflect-metadata doesn't appear to be written in CJS, but also doesn't appear to be a valid ES module (i.e. it doesn't have "type": "module" or an .mjs extension for the entry point). Please contact the package author to fix.

[rbuckton]

reflect-metadata doesn't appear to be written in CJS, but also doesn't appear to be a valid ES module (i.e. it doesn't have "type": "module" or an .mjs extension for the entry point). Please contact the package author to fix.

reflect-metadata is actually written as a Script, not a Module. It does not use import or export (which would imply ESM/MJS), nor does it use module, exports, or require() (which would imply CJS). Instead, it is written so as to augment the global Reflect object.

Since it is written as a Script, and has no syntactic or runtime dependencies on a specific module syntax, it is both a perfectly legal CJS module and a perfectly legal ES module (though NodeJS will only treat it as CJS). It can be loaded in any of the following ways:

• In NodeJS/CommonJS via require("reflect-metadata")
• In NodeJS/ESM via import "reflect-metadata" (because a NodeJS module can import a CJS file)
• In the Browser via <script src="path/to/reflect_metadata/Reflect.js"></script> (because it's a Script)
• In the Browser/ESM via import "path/to/reflect-metadata/Reflect.js"

This package obeys the mechanics of NodeJS's module resolution algorithm. If you are using a bundler that is unable to bundle this package, then it is not obeying the mechanics of NodeJS's module resolution algorithm, and thus the bug lies in the bundler. I would advise you open an issue in your bundler's repo.

[tobiasdiez]

@rbuckton I agree with your view, but could you please add an exports config in package.json. This would help bundlers to understand the nature of the package and is recommended practice anyway: https://nodejs.org/api/packages.html#conditional-exports

[ljharb]

Bundlers should be perfectly capable of figuring out the nature of the package without the exports field; browserify's been doing it for a decade. If newer tools can't keep up with standards that old and time-tested, you may want to avoid them.

[alessiostalla]

@ljharb you can't avoid them if the framework you're using mandates them. For example, Angular warns that

⠸ Building...▲ [WARNING] Module 'reflect-metadata' used by '../node_modules/@strumenta/tylasu/dist/esm/model/model.js' is not ESM

While the above isn't blocking, it will still cause concerned devs to spend time looking for a cause and, hopefully, end up here. When that would be easily avoided by a couple of declarations in package.json, I don't see why they shouldn't be added.

[ljharb]

Sounds like a flaw/mistake/bug in angular then.

[jycouet]

Each time I see an update on this ticket I have mixed feelings (I know, no one cares 😅)

The root cause is this small thing called vite/rollup that all most frameworks are using. (last one coming was Angular 🎉)

This one change in this library is not solving the root cause yes, but solving all warnings.
Not everyone is as good as you @ljharb!