improve implementation

Author: mshima

src/loaders.ts

@@ -35,28 +35,27 @@ type resolve = (
 	recursiveCall?: boolean,
 ) => MaybePromise<Resolved>;
 
-const extensions = ['.js', '.json', '.ts', '.tsx', '.jsx'] as const;
-
-const mappedExtensions = {
-	'.js': ['.ts'],
+const mappedExtensions: Record<string, string[]> = {
+	'.js': ['.ts', '.tsx', 'jsx'],
 	'.cjs': ['.cts'],
 	'.mjs': ['.mts'],
-} as const;
+	'': ['.js', '.json', '.ts', '.tsx', '.jsx'],
+};
+
+function resolveTsAlternativePaths(specifier: string) {
+	const specifierExtension = path.extname(specifier);
+	const extensionsToTry = mappedExtensions[specifierExtension] ?? [];
+	specifier = specifier.slice(0, -specifierExtension.length);
+	return extensionsToTry.map(extension => `${specifier}${extension}`);
+}
 
 async function tryExtensions(
 	specifier: string,
 	context: Context,
 	defaultResolve: resolve,
 ) {
 	let error;
-	const extension = path.extname(specifier);
-	let extensionsToTry = mappedExtensions[extension];
-	if (extensionsToTry) {
-		specifier = specifier.slice(0, -extension.length);
-	} else {
-		extensionsToTry = extensions;
-	}
-	for (const extension of extensionsToTry) {
+	for (const extension of resolveTsAlternativePaths(specifier)) {
 		try {
 			return await resolve(
 				specifier + extension,