example-loader.mjs

import url from 'url';
import path from 'path';
import process from 'process';

const builtins = new Set(
  Object.keys(process.binding('natives')).filter((str) =>
    /^(?!(?:internal|node|v8)\/)/.test(str))
);
const JS_EXTENSIONS = new Set(['.js', '.mjs']);

const baseURL = new url.URL('file://');
baseURL.pathname = process.cwd() + '/';


export default ({ resolve: parentResolve }) => {
  return {
    resolve(specifier, parentModuleURL = baseURL) {
      if (builtins.has(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.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}.`);
      }
      return {
        url: resolved.href,
        format: 'esm'
      };
    }
  };
}