express-resolver.js

 const Path = require('path');

/* fix paths before nodejs-like es6 imports
 * 'root' must be served by statics middleware
 * 'mount' must be a path prefix to a folder e.g. /modules/
 * To allow a node module 'abc' to be served through this,
 * place a symlink from node_modules/abc to `${root}${mount}abc`
 * (this is not automatic for security reasons)
 */
module.exports = function(root, mount) {
	if (!mount.startsWith('/') && !mount.endsWith('/')) {
		throw new Error("Mount must start and end with a slash");
	}
	return async function esm(req, res, next) {
		if (!req.path.startsWith(mount)) return next();
		const lookup = req.path.substring(mount.length);
		const module = require.resolve(lookup);
		if (!module) return next();
		const path = Path.relative(".", module).replace(/^node_modules\//, mount);
		req.url = path;
		next();
	};
};