Loader for Node ESM support.

js-ext.loader.mjs

import path from 'path'
import process from 'process'
import Module from 'module'

/**
 *
 * # Experimental Modules Loader
 *
 * @see {@link https://nodejs.org/api/esm.html#esm_loader_hooks}
 *
 *
 * With this loader, running:
 *
 * @example `NODE_OPTIONS='--experimental-modules --loader ./js-ext.loader.mjs' node x.js`
 *
 * A dummy loader to load JavaScript restricted
 * to browser resolution rules with only JS file extension
 * and Node.js builtin modules support could be written:
 */

const builtins = Module.builtinModules
const JS_EXTENSIONS = new Set(['.js', '.mjs'])

const baseURL = new URL('file://')
baseURL.pathname = `${process.cwd()}/`

export function resolve (specifier, parentModuleURL = baseURL, defaultResolve) {
  if (builtins.includes(specifier)) {
    return {
      url: specifier,
      format: 'builtin'
    }
  }
  if (/^\.{0,2}[/]/.test(specifier) !== true && !specifier.startsWith('file:')) {
    // For node_modules support:
    // return defaultResolve(specifier, parentModuleURL);
    throw new Error(
      `imports must begin with '/', './', or '../'; '${specifier}' does not`)
  }
  const resolved = new URL(specifier, parentModuleURL)
  const ext = path.extname(resolved.pathname)
  if (!JS_EXTENSIONS.has(ext)) {
    throw new Error(
      `Cannot load file with non-JavaScript file extension ${ext}.`)
  }
  console.count('loader resolved')
  return {
    url: resolved.href,
    format: 'esm'
  }
}

package.json

{
  "name": "js-ext-loader",
  "version": "1.0.1",
  "bin": "js-ext.loader.mjs",
  "description": "To exec and load ESM files with '.js' ext in addition to '.mjs'.",
  "main": "js-ext.loader.mjs",
  "module": "js-ext.loader.mjs",
  "scripts": {
    "test":"NODE_OPTIONS='--experimental-modules --loader ./js-ext.loader.mjs' node test.js"
  },
  "repository": {
    "type": "git",
    "url": "git+https://gist.github.com/TravisMullen/16dfabca0db4bb4c32fb4f5ef297b25a"
  },
  "author": "Travis Mullen",
  "license": "ISC"
}

test.js

import {
  platform,
  arch,
  cpus,
  userInfo
} from 'os'
import {
  inspect
} from 'util'

for (const method of [platform, arch, cpus, userInfo]) {
  
  console.assert(typeof(method) !== 'undefined', 'modules should be loaded and defined.')
  
  const test = method()
  console.assert(typeof(test) !== 'undefined', 'module methods should return data.')
  console.log(inspect(test, { colors: true }))

}