Provide a way for CommonJS scripts to synchronously import ESM

[dfabulich]

Today, it's straightforward to take a CJS library and add an ESM wrapper around it; this is recommended practice in https://github.com/nodejs/node/blob/master/doc/api/esm.md

// ./node_modules/pkg/wrapper.mjs
import cjsModule from './index.cjs';
export const name = cjsModule.name;

But, as far as I can tell, there's no way to write a synchronous CJS wrapper around an ESM library. CJS scripts can only import ESM with asynchronous dynamic import.

I believe I understand why require('./index.mjs') wouldn't match the CJS require semantics, but I think it should be possible in principle to implement a module.importSync() feature that would work like this:

// ./node_modules/pkg/wrapper.cjs
const {importSync} = require('module');
const {name} = importSync('./index.mjs');
module.exports.name = name;

This feature would be useful, because it would provide a path for a CJS library to port to ESM while preserving CJS semantics. A typical flow might be:

1. Pure CJS library
2. Add ESM wrapper and exports map in package.json
3. Port to ESM, add CJS wrapper, update exports map

[GeoffreyBooth]

I think that your importSync would run into the same issues as “require of ESM” which has had a lot of discussion in this repo and a PR from @weswigham. See #454.

[dfabulich]

Thanks, I think I get it now. (The core problem is that index.mjs may or may not use top-level await.)