index.mjs

import __dirname from './dirname';
import util from 'util';
const {promisify} = util;
import fs from 'fs';
const asyncStat = promisify(fs.stat);
const asyncReadFile = promisify(fs.readFile);
import url from 'url';
const {URL} = url;
const dirHref = `file://${__dirname}/`;
const overloadsHref = `${dirHref}overloads/`;

// you can do any global mutation here
// setup async_hooks etc.
console.error('initializing loader... (do setup here)');

// this will find the package.json URL associated with a given file URL
const grabPackage = async (url) => {
  try {
    return JSON.parse(
      await asyncReadFile(new URL('./package.json', url)));
  } catch (e) {
    if (e.code !== 'ENOENT') {
      throw e;
    }
  }
  const parent = new URL('../', url);
  if (parent.href === url) {
    return undefined;
  }
  return grabPackage(parent.href);
}

const skipPackageSearch = new Set(['fs']);
const overloads = {
  __proto__: null,
  fs() {
    return new URL(
      `./fs.mjs`,
      overloadsHref
    ).href;
  },
  foo({url: resolved}, pkg) {
    if (pkg.version === '0.0.0') {
      return new URL(
        `./foo@0.0.0.mjs`,
        overloadsHref
      ).href;
    } if (pkg.version === '1.0.0') {
      return new URL(
        `./foo@1.0.0.mjs`,
        overloadsHref
      ).href;
    } else {
      return resolved;
    }
  }
}
export async function resolve(specifier, parentModuleURL, defaultResolver) {
  if (
    new URL(parentModuleURL).href.indexOf(overloadsHref) == 0) {
    return defaultResolver(specifier, parentModuleURL);
  }
  try {
    const overload = overloads[specifier];
    if (overload) {
      if (skipPackageSearch.has(specifier)) {
        return defaultResolver(overload(), parentModuleURL);
      } else {
        const resolved = await defaultResolver(
          specifier,
          parentModuleURL
        );
        const pkg = await grabPackage(resolved.url);
        return defaultResolver(overload(resolved, pkg), parentModuleURL);
      }
    }
  } catch (e) {
    console.trace(e);
  }
  return defaultResolver(specifier, parentModuleURL);
}