Add support for named imports in ESM

[shadowspawn]

Pull Request

Problem

Importing commander as CommonJs into an ECMAScript module only allows using the default import, and can not use named imports.

// import { program } from 'commander'; // ERROR
import commander from 'commander';
const program = commander.program;

Related issue: #1284

Solution

Add esm wrapper and use as a deep import:

import { program } from 'commander/esm.mjs';

In the future we will add support so the deep import is not needed by using conditional exports and/or subpaths. Being conservative for now as the functionality is still marked as experimental in node 12. (There have been breaking changes in the support in minor versions of node 12 and 13.)

• https://nodejs.org/docs/latest-v12.x/api/packages.html#packages_conditional_exports
• https://nodejs.org/docs/latest-v12.x/api/packages.html#packages_subpath_exports

The --experimental-modules option used in the test-esm run-script will get dropped by node eventually, but currently allows that test to run locally with node 10 through node 15.

ChangeLog

• added support for named imports from ECMAScript modules

[abetomo]

LGTM

[shadowspawn]

So fast! 🚀
Thanks @abetomo

[cravler]

looks like a typo, must be esm.mjs

[shadowspawn]

Oh, good catch, thanks! I changed the name half way through.

[shadowspawn]

(For interest, I thought wrapper was a suitable name for a wrapper used with conditional exports and not seen by normal client use. But when I changed to deep import and the name became visible, I wanted something more meaningful.)

[shadowspawn]

Fixed in #1443