alias-loader.mjs

import path from 'path';
import { pathToFileURL } from 'url';

const npmPackage = await import(
	pathToFileURL(`${process.cwd()}/package.json`).href
).then(({ default: json }) => json);

const aliases = (() => {
	const base = process.cwd();
	const { aliases } = npmPackage || {};

	const absoluteAliases = Object.create(null);

	for (
		let aliasKeys = Object.keys(aliases),
			i = 0,
			end = aliasKeys.length;
		i < end;
		i++
	) {
		const aliasKey = aliasKeys[i];
		const aliasPath = aliases[aliasKey];

		if (aliasPath[0] !== '/') {
			absoluteAliases[aliasKey] = path.join(base, aliasPath);
		}
	}

	return absoluteAliases;
})();
const aliasKeys = Object.keys(aliases);

const isAliasInSpecifier = (path, alias) => {
	const aliasLength = alias.length;

	return (
		path.indexOf(alias) === 0
		&& (
			path.length === aliasLength
			|| path[aliasLength] === '/'
		)
	);
};

export const resolve = (
	specifier,
	parentModuleURL,
	defaultResolve,
) => {
	const aliasKey = aliasKeys.find((key) => isAliasInSpecifier(specifier, key));

	if (aliasKey) {
		const url = path.join(aliases[aliasKey], specifier.substr(aliasKey.length));

		return { url };
	}

	return defaultResolve(specifier, parentModuleURL, defaultResolve);
};

// package.json
{
  "aliases": {
    "~": "./src"
  }
}

$> node --experimental-loader=./alias-loader.mjs ./index.mjs

$> mocha --recursive ./src --experimental-loader=./alias-loader.mjs

⚠️ This appears to have broken somewhere between node v15.1.0 and v15.6.0. I'm suddenly getting an error:

SyntaxError: Setter must have exactly one formal parameter.
    at Loader.moduleStrategy (node:internal/modules/esm/translators:147:18)

however, there is no setter on translators.js:147

The above was due to an error in my own source-code, which node ESM erroneously reported as coming from the alias loader