[2.3.0+] TypeError: Unknown file extension ".json" for /...package.json

[jrgleason]

• Rollup Version: latest
• Operating System (or Browser): OSX
• Node Version (if applicable): v14.0.0-pre
• Link to reproduction (IMPORTANT, read below): https://gist.github.com/jrgleason/06e054f1768260a287884a83609f1e2a

Expected Behavior

Pre-2.3.0 (IE 2.2.0) I could just run rollup -c rollup.config.mjs and everything would work.

Actual Behavior

After 2.3.0 I get...

[!] TypeError: Unknown file extension ".json" for /.../fail-rollup/package.json

Work Around

Explicitly use version 2.2.0 in my package.json

[lukastaegert]

Either change your file extension to .js, configure Node to use —experimental-json-modules or use a way to import .json files that is actually supported for .mjs files in Node, cf. https://rollupjs.org/guide/en/#using-untranspiled-config-files

Our assumption is that if you are using .mjs, you are doing so because you want your code to follow Node 13+ ES module semantics, which will now be enforced by loading your config as a Node ES module.

[lukastaegert]

Here is another way to fix this: https://nodejs.org/api/esm.html#esm_commonjs_json_and_native_modules

[jrgleason]

Can you speak a little more about that assumption and how that changed from 2.2.0? I guessed that this was a change to better conform to standards but I am confused about why it worked and then stopped? I am using node 14 in 2.2 and it is working fine. So I am just confused about how the assumptions changed. Is there a release note that might help me better understand the motive better?

[jrgleason]

I tried using the code in my project (more advanced than the example I gave)...

node --experimental-json-modules ./node_modules/.bin/rollup --config rollup.config.mjs

But again with no code changes I now get...

[!] SyntaxError: The requested module 'rollup-plugin-eslint' does not provide an export named 'eslint'
file:///.../ui/rollup.config.mjs:4
import { eslint } from "rollup-plugin-eslint";

Which again works fine when I downgrade to 2.2

[jrgleason]

Seems to be with all the plugins I commented out lint and now I get...

import { terser } from "rollup-plugin-terser";
[!] SyntaxError: The requested module 'rollup-plugin-terser' does not provide an export named 'terser'

[lukastaegert]

It is all explained in the documentation I linked. Prior to 2.3, all configs were transpiled to CJS before they were executed, now they are not if you are using Node 13+. Before 2.3, we did not try to replicate Node ESM semantics also because they were somewhat unstable at that time. Most plugins, including terser, are only available as CJS. If a Node ES module imports a CJS module, it will only receive a default export. In case of e.g. terser, you could

import terser from 'terser';
...
terser.terser()

[jrgleason]

When I remove both plugins it does seem to work.

[lukastaegert]

By the way, it is actually not true what I said earlier that those plugins are only available as CJS, they are just not configured to provide an ES module when they are imported from a Node ES module. They would need to use conditional exports to make that work, just like Rollup itself does it now.

[jrgleason]

Right and just for the record the working example was...

import rollupTerser from "rollup-plugin-terser";
rollupTerser.terser()

Just cause the dependency was wrong and didn't want to confuse people. Or am I wrong and I should be using the terser library directly?

[lukastaegert]

No, you are right, it should of course import from the plugin 🙄

[wayfarer3130]

In the config files, you can always do:

import { readFileSync } from "fs";
const pkg = JSON.parse(readFileSync('package.json', {encoding: 'utf8'}));