fix(repack): wrap babel-loader fallback value in an array (#1456)

Co-authored-by: Daniel Williams <dannyhyunsoowilliams@gmail.com>

Author: ilteoood

.changeset/babel-loader-resolve-loader-fallback-array.md

@@ -0,0 +1,5 @@
+---
+"@callstack/repack": patch
+---
+
+Fix `BabelPlugin` to set the `babel-loader` entry of `resolveLoader.fallback` as an array containing the resolved path, instead of a plain string. Rspack's resolver and downstream tools that consume the resolved loader config (e.g. `RSDoctor`) expect the value to be an array; a bare string triggered `Given napi value is not an array on NapiResolveOptions.fallback`. This still matches Rspack's `ResolveAlias` (`{ [x: string]: string | false | (string | false)[] }`) and Webpack's resolver loader fallback shape.

packages/repack/src/plugins/BabelPlugin.ts

@@ -1,5 +1,6 @@
 import type { Compiler as RspackCompiler } from '@rspack/core';
 import type { Compiler as WebpackCompiler } from 'webpack';
+
 /**
  * Plugin that adds babel-loader fallback to resolveLoader configuration.
  * This ensures babel-loader can be resolved regardless of the package manager used,
@@ -18,7 +19,7 @@ export class BabelPlugin {
       ...compiler.options.resolveLoader,
       fallback: {
         ...compiler.options.resolveLoader?.fallback,
-        'babel-loader': require.resolve('babel-loader'),
+        'babel-loader': [require.resolve('babel-loader')],
       },
     };
   }

packages/repack/src/plugins/__tests__/BabelPlugin.test.ts

@@ -0,0 +1,30 @@
+import type { Compiler } from '@rspack/core';
+import { BabelPlugin } from '../BabelPlugin.js';
+
+const compilerMock: { options: Compiler['options'] } = {
+  options: {} as Compiler['options'],
+};
+
+describe('BabelPlugin', () => {
+  it('sets resolveLoader.fallback[babel-loader] as an array entry', () => {
+    const pluginInstance = new BabelPlugin();
+    pluginInstance.apply(compilerMock as unknown as Compiler);
+
+    expect(compilerMock.options.resolveLoader?.fallback).toEqual({
+      'babel-loader': [expect.any(String)],
+    });
+  });
+
+  it('preserves existing record fallback entries', () => {
+    compilerMock.options.resolveLoader = {
+      fallback: { 'foo-loader': '/path/to/foo-loader' },
+    };
+    const pluginInstance = new BabelPlugin();
+    pluginInstance.apply(compilerMock as unknown as Compiler);
+
+    expect(compilerMock.options.resolveLoader.fallback).toEqual({
+      'foo-loader': '/path/to/foo-loader',
+      'babel-loader': [expect.any(String)],
+    });
+  });
+});