fix: should add index main file when redirect.js.path set to false

packages/core/src/config.ts

@@ -63,6 +63,7 @@ import {
   calcLongestCommonPath,
   checkMFPlugin,
   getAbsolutePath,
+  isDirectory,
   isEmptyObject,
   isIntermediateOutputFormat,
   isObject,
@@ -1436,6 +1437,19 @@ const composeBundlelessExternalConfig = (
                 } else {
                   // 1. js files hit JS_EXTENSIONS_PATTERN,./foo ->./foo.mjs
                   if (jsRedirectExtension) {
+                    // If the import path refers to a directory,
+                    // it most likely actually refers to a `index.*` file due to Node's module resolution.
+                    // When redirect.js.path is set to false, index should still be added before adding extension.
+                    // When redirect.js.path is true, the resolver directly generate correct resolvedRequest with index appended.
+                    if (
+                      !jsRedirectPath &&
+                      (await isDirectory(
+                        join(dirname(issuer), resolvedRequest),
+                      ))
+                    ) {
+                      // This uses `/` instead of `path.join` here because `join` removes potential "./" prefixes
+                      resolvedRequest = `${resolvedRequest.replace(/\/+$/, '')}/index`;
+                    }
                     resolvedRequest = `${resolvedRequest}${jsExtension}`;
                   }
                 }

packages/core/src/utils/helper.ts

@@ -246,3 +246,12 @@ const windowsSlashRegex = /\\/g;
 export function normalizeSlash(p: string): string {
   return p.replace(windowsSlashRegex, '/');
 }
+
+export async function isDirectory(filePath: string): Promise<boolean> {
+  try {
+    const stat = await fsP.stat(filePath);
+    return stat.isDirectory();
+  } catch {
+    return false;
+  }
+}

tests/integration/redirect/js.test.ts

@@ -53,11 +53,11 @@ test('redirect.js.path false', async () => {
     import { bar } from "@/bar";
     import { foo } from "@/foo";
     import { baz } from "~/baz";
-    import { bar as external_bar_js_bar } from "./bar.js";
+    import { bar as index_js_bar } from "./bar/index.js";
     import { foo as external_foo_js_foo } from "./foo.js";
     export * from "./.hidden.js";
-    export * from "./.hidden-folder.js";
-    const src = lodash.toUpper(lodash_merge(external_foo_js_foo) + external_bar_js_bar + foo + bar + baz + typeof prettier.version);
+    export * from "./.hidden-folder/index.js";
+    const src = lodash.toUpper(lodash_merge(external_foo_js_foo) + index_js_bar + foo + bar + baz + typeof prettier.version);
     export { src as default };
     "
   `);