add currentData property to hook results. (#1500)

Author: phryneas

packages/toolkit/src/query/react/buildHooks.ts

@@ -306,6 +306,12 @@ export type UseQueryStateResult<
 
 type UseQueryStateBaseResult<D extends QueryDefinition<any, any, any, any>> =
   QuerySubState<D> & {
+    /**
+     * Where `data` tries to hold data as much as possible, also re-using
+     * data from the last arguments passed into the hook, this property
+     * will always contain the received data from the query, for the current query arguments.
+     */
+    currentData?: ResultTypeFrom<D>
     /**
      * Query has not started yet.
      */
@@ -342,11 +348,21 @@ type UseQueryStateDefaultResult<D extends QueryDefinition<any, any, any, any>> =
         | { isLoading: true; isFetching: boolean; data: undefined }
         | ({
             isSuccess: true
-            isFetching: boolean
+            isFetching: true
             error: undefined
           } & Required<
             Pick<UseQueryStateBaseResult<D>, 'data' | 'fulfilledTimeStamp'>
           >)
+        | ({
+            isSuccess: true
+            isFetching: false
+            error: undefined
+          } & Required<
+            Pick<
+              UseQueryStateBaseResult<D>,
+              'data' | 'fulfilledTimeStamp' | 'currentData'
+            >
+          >)
         | ({ isError: true } & Required<
             Pick<UseQueryStateBaseResult<D>, 'error'>
           >)
@@ -421,6 +437,7 @@ const queryStatePreSelector = (
   return {
     ...currentState,
     data,
+    currentData: currentState.data,
     isFetching,
     isLoading,
     isSuccess,

packages/toolkit/src/query/tests/unionTypes.test.ts

@@ -115,6 +115,20 @@ describe.skip('TS only tests', () => {
       expectExactType(false as false)(result.isError)
     }
 
+    expectExactType('' as string | undefined)(result.currentData)
+    // @ts-expect-error
+    expectExactType('' as string)(result.currentData)
+
+    if (result.isSuccess) {
+      if (!result.isFetching) {
+        expectExactType('' as string)(result.currentData)
+      } else {
+        expectExactType('' as string | undefined)(result.currentData)
+        // @ts-expect-error
+        expectExactType('' as string)(result.currentData)
+      }
+    }
+
     // @ts-expect-error
     expectType<never>(result)
     // is always one of those four