Only transform files that need to be transformed

src/index.ts

@@ -60,12 +60,19 @@ function transformer(
 
 	let code = fs.readFileSync(filePath, 'utf8');
 
+	const hasImportOrExport = /^\s*(?:import|export)\s/gm;
+
 	if (filePath.endsWith('.cjs') && nodeSupportsImport) {
 		const transformed = transformDynamicImport(filePath, code);
 		if (transformed) {
 			code = applySourceMap(transformed, filePath);
 		}
-	} else {
+	} else if (
+		// Best guesses for files that need to be transformed.
+		!filePath.endsWith('.js') // not .js, so possibly ts, tsx, mjs, mts etc.
+		|| code.includes('import(') // dynamic import
+		|| hasImportOrExport.test(code) // any line starts with import or export
+	) {
 		const transformed = transformSync(
 			code,
 			filePath,
@@ -75,6 +82,8 @@ function transformer(
 		);
 
 		code = applySourceMap(transformed, filePath);
+	} else {
+		// Skipped, we expect it not to need to be transformed.
 	}
 
 	module._compile(code, filePath);