fix: inherit source type from babel

.changeset/itchy-deer-allow.md

@@ -0,0 +1,5 @@
+---
+"@callstack/repack": patch
+---
+
+Babel can detect whether a file being transformed is ESM or CJS and return this information. However, SWC previously assumed it was transforming only ESM files, which could cause issues with default export interop in some cases. SWC will now inherit the source type from Babel.

packages/repack/src/loaders/babelLoader/babelLoader.ts

@@ -1,4 +1,5 @@
 import {
+  type BabelFileResult,
   type TransformOptions,
   loadOptions,
   parseSync,
@@ -63,11 +64,15 @@ function buildBabelConfig(
   return babelConfig;
 }
 
+type BabelTransformResult = BabelFileResult & {
+  sourceType: 'script' | 'module';
+};
+
 export const transform = async (
   src: string,
   transformOptions: TransformOptions,
   customOptions?: CustomTransformOptions
-) => {
+): Promise<BabelTransformResult> => {
   const babelConfig = buildBabelConfig(transformOptions, {
     includePlugins: customOptions?.includePlugins,
     excludePlugins: customOptions?.excludePlugins,
@@ -100,7 +105,12 @@ export const transform = async (
     throw new Error(`Failed to transform source file: ${babelConfig.filename}`);
   }
 
-  return result;
+  const sourceType = sourceAst.program.sourceType;
+
+  return {
+    ...result,
+    sourceType,
+  };
 };
 
 export default async function babelLoader(

packages/repack/src/loaders/babelSwcLoader/babelSwcLoader.ts

@@ -133,6 +133,7 @@ export default async function babelSwcLoader(
         lazy: lazyImports,
         type: swcConfig.module!.type,
       },
+      isModule: babelResult.sourceType === 'module',
     };
 
     const swcResult = swc.transformSync(babelResult?.code!, {