fix(broadcast-client): don't overwrite an existing query's data on an incoming 'added' message

.changeset/fix-broadcast-added-overwrites-existing-data.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/query-broadcast-client-experimental': patch
+---
+
+fix: don't overwrite an existing query's resolved data when another tab broadcasts an `added` event for it

packages/query-broadcast-client-experimental/src/__tests__/index.test.ts

@@ -122,6 +122,63 @@ describe('broadcastQueryClient', () => {
 
       expect(mockPostMessage).toHaveBeenCalled()
     })
+
+    it('should not overwrite an existing query with data when an "added" message arrives for it', () => {
+      const existingKey = queryKey()
+
+      broadcastQueryClient({
+        queryClient,
+        broadcastChannel: 'test_channel',
+      })
+
+      // A query that already resolved in this tab (e.g. it fetched before
+      // another tab mounted the same key).
+      queryClient.setQueryData(existingKey, { value: 'resolved' })
+      const existingQuery = queryCache.find({ queryKey: existingKey })!
+
+      // Another tab just mounted the same key for the first time, so its
+      // `build()` broadcasts an `added` message with a pending state and
+      // no data.
+      lastCreatedChannel.onmessage?.({
+        type: 'added',
+        queryHash: existingQuery.queryHash,
+        queryKey: existingKey,
+        state: { status: 'pending', data: undefined },
+      })
+
+      expect(queryClient.getQueryData(existingKey)).toEqual({
+        value: 'resolved',
+      })
+      expect(queryClient.getQueryState(existingKey)?.status).toBe('success')
+    })
+
+    it('should adopt an "added" message\'s state for an existing query that has no data yet', () => {
+      const existingKey = queryKey()
+
+      broadcastQueryClient({
+        queryClient,
+        broadcastChannel: 'test_channel',
+      })
+
+      // This tab has already built the query (e.g. an observer mounted it)
+      // but hasn't fetched it yet.
+      const existingQuery = queryCache.build(queryClient, {
+        queryKey: existingKey,
+      })
+
+      // Another tab already had this key resolved (e.g. via `initialData`)
+      // when it mounted, so its `added` message carries real data.
+      lastCreatedChannel.onmessage?.({
+        type: 'added',
+        queryHash: existingQuery.queryHash,
+        queryKey: existingKey,
+        state: { status: 'success', data: { value: 'from other tab' } },
+      })
+
+      expect(queryClient.getQueryData(existingKey)).toEqual({
+        value: 'from other tab',
+      })
+    })
   })
 
   describe('postMessage error handling', () => {

packages/query-broadcast-client-experimental/src/index.ts

@@ -175,7 +175,9 @@ export function broadcastQueryClient({
         }
       } else if (type === 'added') {
         if (query) {
-          query.setState(state)
+          if (query.state.data === undefined) {
+            query.setState(state)
+          }
           return
         }
         queryCache.build(