Add a failing tests (and update one test)

Currently, useSelector(selector) will call the passed selector twice on
mount when it only needs to be called once on mount. This is
unnecessary, and in the case of expensive selectors, can be a
performance concern.

Author: sufian-slack

test/hooks/useSelector.spec.js

@@ -45,14 +45,14 @@ describe('React', () => {
           })
 
           expect(result.current).toEqual(0)
-          expect(selector).toHaveBeenCalledTimes(2)
+          expect(selector).toHaveBeenCalledTimes(1)
 
           act(() => {
             store.dispatch({ type: '' })
           })
 
           expect(result.current).toEqual(1)
-          expect(selector).toHaveBeenCalledTimes(3)
+          expect(selector).toHaveBeenCalledTimes(2)
         })
       })
 
@@ -246,6 +246,39 @@ describe('React', () => {
 
           expect(renderedItems.length).toBe(1)
         })
+
+        it('calls selector exactly once on mount and on update', () => {
+          store = createStore(({ count } = { count: 0 }) => ({
+            count: count + 1,
+          }))
+
+          let numCalls = 0
+          const selector = (s) => {
+            numCalls += 1
+            return s.count
+          }
+          const renderedItems = []
+
+          const Comp = () => {
+            const value = useSelector(selector)
+            renderedItems.push(value)
+            return <div />
+          }
+
+          rtl.render(
+            <ProviderMock store={store}>
+              <Comp />
+            </ProviderMock>
+          )
+
+          expect(numCalls).toBe(1)
+          expect(renderedItems.length).toEqual(1)
+
+          store.dispatch({ type: '' })
+
+          expect(numCalls).toBe(2)
+          expect(renderedItems.length).toEqual(2)
+        })
       })
 
       it('uses the latest selector', () => {