Fix cleared loaderData bug on thrown action errors

.changeset/hip-laws-teach.md

@@ -0,0 +1,5 @@
+---
+"react-router": patch
+---
+
+Fix bug where bubbled action errors would result in `loaderData` being cleared at the handling `ErrorBoundary` route

packages/react-router/__tests__/router/should-revalidate-test.ts

@@ -1170,4 +1170,66 @@ describe("shouldRevalidate", () => {
 
     router.dispose();
   });
+
+  it("preserves ancestor loaderData on bubbled action errors when not revalidating with a custom data strategy", async () => {
+    let history = createMemoryHistory();
+    let router = createRouter({
+      history,
+      routes: [
+        {
+          id: "root",
+          path: "/",
+          hasErrorBoundary: true,
+          loader: () => "NOPE",
+          children: [
+            {
+              id: "index",
+              index: true,
+              action: () => {
+                throw new Response("ERROR 400", { status: 400 });
+              },
+            },
+          ],
+        },
+      ],
+      hydrationData: {
+        loaderData: {
+          root: "ROOT",
+        },
+      },
+      async dataStrategy({ request, matches }) {
+        let keyedResults = {};
+        let matchesToLoad = matches.filter((match) =>
+          match.unstable_shouldCallHandler(
+            request.method === "POST"
+              ? undefined
+              : !match.unstable_shouldRevalidateArgs?.actionStatus ||
+                  match.unstable_shouldRevalidateArgs.actionStatus < 400
+          )
+        );
+        await Promise.all(
+          matchesToLoad.map(async (match) => {
+            keyedResults[match.route.id] = await match.resolve();
+          })
+        );
+        return keyedResults;
+      },
+    });
+    router.initialize();
+
+    router.navigate("/?index", {
+      formMethod: "post",
+      formData: createFormData({ gosh: "dang" }),
+    });
+    await tick();
+    expect(router.state).toMatchObject({
+      location: { pathname: "/" },
+      navigation: { state: "idle" },
+      loaderData: { root: "ROOT" },
+      actionData: null,
+      errors: { root: new ErrorResponseImpl(400, "", "ERROR 400") },
+    });
+
+    router.dispose();
+  });
 });

packages/react-router/lib/router/router.ts

@@ -686,7 +686,15 @@ interface ShortCircuitable {
   shortCircuited?: boolean;
 }
 
-type PendingActionResult = [string, SuccessResult | ErrorResult];
+// Track any pending errors from the action (or other pre-loader flows).
+// The format is [bubbledRouteId, result, actionRouteId?]
+// We should probably change this to an object now that we (optionally) track
+// the original action route id so we can clear out loaderData in the right in
+// processRouteLoaderData
+//
+type PendingActionResult =
+  | [string, SuccessResult | ErrorResult]
+  | [string, SuccessResult | ErrorResult, string];
 
 interface HandleActionResult extends ShortCircuitable {
   /**
@@ -1858,7 +1866,11 @@ export function createRouter(init: RouterInit): Router {
 
       return {
         matches,
-        pendingActionResult: [boundaryMatch.route.id, result],
+        pendingActionResult: [
+          boundaryMatch.route.id,
+          result,
+          actionMatch.route.id,
+        ],
       };
     }
 
@@ -6047,11 +6059,13 @@ function processRouteLoaderData(
   });
 
   // If we didn't consume the pending action error (i.e., all loaders
-  // resolved), then consume it here.  Also clear out any loaderData for the
-  // throwing route
+  // resolved), then consume it here
   if (pendingError !== undefined && pendingActionResult) {
     errors = { [pendingActionResult[0]]: pendingError };
-    loaderData[pendingActionResult[0]] = undefined;
+    // Clear out any loaderData for the throwing route
+    if (pendingActionResult[2]) {
+      loaderData[pendingActionResult[2]] = undefined;
+    }
   }
 
   return {