Add an option for self-registering

lib/create-esm-loader.js

@@ -1,12 +1,24 @@
 // # create-esm-loader.js
 import path from 'node:path';
+import module from 'node:module';
 import { isMainThread } from 'node:worker_threads';
 
-console.log(isMainThread);
-
 // # createLoader(config)
 export default function createLoader(config = {}) {
 
+	// If we're running from the main thread, then we just export a `register` 
+	// method that registers the loader *itself*.
+	if (isMainThread && (config.register || typeof config === 'string')) {
+		const url = typeof config === 'string' ? config : config.register;
+		return {
+			register(options) {
+				module.register(url, {
+					data: options,
+				});
+			},
+		};
+	}
+
 	// Bind the resolve and load hooks from the config object to the hooks 
 	// object we'll be exporting. If they were not specified, we do nothing.
 	const hooks = {};

test/loaders-test.js

@@ -37,6 +37,12 @@ describe('ESM loaders', function() {
 				`data:text/javascript,import {register} from "node:module"; register(${JSON.stringify(url)});`,
 			);
 		};
+		this.register = function(specifier, options = {}) {
+			const url = new URL(specifier, import.meta.url).href;
+			return this.run(
+				`data:text/javascript,import {register} from ${JSON.stringify(url)};register(${JSON.stringify(options)})`,
+			);
+		};
 	});
 
 	it('an http loader', async function() {
@@ -72,6 +78,20 @@ describe('ESM loaders', function() {
 
 	});
 
+	it('a self-registering loader', async function() {
+
+		const run = this.register('./loaders/self-register.js', {
+			extension: '.ts',
+			loader: 'ts',
+		});
+		let result = await run(`
+		import fn from './files/fn.ts';
+		export default fn('I like TypeScript');
+		`);
+		expect(result).to.equal('I don\'t like TypeScript');
+
+	});
+
 	it('a composite loader', async function() {
 
 		const server = new http.Server((req, res) => {

test/loaders/self-register.js

@@ -0,0 +1,20 @@
+// # self-register.js
+import create from 'create-esm-loader';
+
+export const { register, load, initialize } = create({
+	register: import.meta.url,
+	async load(url, ctx, nextLoad) {
+		const { extension } = this.options;
+		if (!url.endsWith(extension)) return nextLoad(url);
+		const { source } = await nextLoad(url, { format: 'module' });
+		const { transform } = await import('esbuild');
+		const { loader = 'js' } = this.options || {};
+		const { code } = await transform(source, {
+			loader,
+		});
+		return {
+			source: code,
+			format: 'module',
+		};
+	},
+});