Cannot import from package which has "type": "module"

[zaripych]

I'm using native ES modules in Node v16.15.0. This is done by specifying "type" prop in package.json:

  "type": "module"

Here are repro steps:

mkdir test-proj
cd ./test-proj
npm init -y
npm pkg set type module
echo 'import swc from "rollup-plugin-swc";console.log("", swc());' > index.js
npm add rollup-plugin-swc
node ./index.js

Would get me something like this:

file:///home/rz/projects/test-proj/index.js:1
import swc from "rollup-plugin-swc";console.log("", swc());
                                                    ^

TypeError: swc is not a function
    at file:///home/rz/projects/test-proj/index.js:1:53
    at ModuleJob.run (node:internal/modules/esm/module_job:198:25)
    at async Promise.all (index 0)
    at async ESMLoader.import (node:internal/modules/esm/loader:385:24)
    at async loadESM (node:internal/process/esm_loader:88:5)
    at async handleMainPromise (node:internal/modules/run_main:61:12)

The swc that we import above is evaluated as { default: [Function: swc] }

So, this happens because the package.json doesn't have the type field, so all .js files are treated as CommonJs even if they use import/export syntax.

This happens because Node.js ignores "module" prop in package.json of the rollup-plugin-swc. As per https://nodejs.org/api/packages.html#dual-commonjses-module-packages.

So, if we rename ./dist/esm/index.js to ./dist/esm/index.mjs and change the package.json of the rollup-plugin-swc to look like below:

  "main": "dist/cjs/index.js",
  "module": "dist/esm/index.mjs",
  "types": "dist/index.d.ts",
  "type": "commonjs",
  "exports": {
    "require": "./dist/cjs/index.js",
    "import": "./dist/esm/index.mjs"
  },

Then it works. I didn't go far to test for backward compatibility with prior versions of Node - but it should work.

@rollup/plugin-node-resolve adopts similar approach but they use extra package.json in es directory:

{"type":"module"}

instead of renaming to .mjs https://github.com/rollup/plugins/blob/master/packages/node-resolve/rollup.config.js#L16

[robiot]

Hack solution

import * as _swc from "rollup-plugin-swc";

const swc = _swc.default.default;