Implement transform loader helper

lib/create-esm-loader.js

@@ -6,15 +6,16 @@ export default function createLoader(config = {}) {
 	// object we'll be exporting. If they were not specified, we do nothing.
 	const hooks = {};
 	const ctx = { options: void 0 };
-	bind(hooks, config, ctx, 'resolve');
-	bind(hooks, config, ctx, 'load');
+	const parsedConfig = normalizeConfig(config);
+	bind(hooks, parsedConfig, ctx, 'resolve');
+	bind(hooks, parsedConfig, ctx, 'load');
 
 	// The initialize hook is a bit different because if it's not specified, 
 	// then we use a default hook.
 	if (typeof config.initialize === 'function') {
 		bind(hooks, config, ctx, 'initialize');
 	} else {
-		config.initialize = options => void (ctx.options = options);
+		hooks.initialize = options => void (ctx.options = options);
 	}
 
 	// Return the hooks at last.
@@ -31,3 +32,32 @@ function bind(hooks, config, ctx, hook) {
 		return fn.call(ctx, ...args);
 	};
 }
+
+// # normalizeConfig(config)
+// Normalizes the configuration, meaning that if we find a `test` property, 
+// we'll automatically generate the corresponding `load()` hook.
+function normalizeConfig(config) {
+	if (config.test) {
+		const { test, transform, load, ...rest } = config;
+		const match = typeof test === 'function' ? test : url => {
+			return test.test(new URL(url).pathname);
+		};
+		return {
+			async load(url, ctx, nextLoad) {
+				if (!match(url)) return nextLoad(url);
+				const { source } = await nextLoad(url, { format: 'module' });
+				const result = await transform(source);
+				return {
+					source: result,
+					format: 'module',
+				};
+			},
+			...rest,
+		};
+	} else {
+
+		// Return as is otherwise.
+		return config;
+
+	}
+}