Add check for undefined middleware in configureStore (#959)

Author: Shrugsy

src/configureStore.test.ts

@@ -68,6 +68,53 @@ describe('configureStore', () => {
     })
   })
 
+  describe('given undefined middleware', () => {
+    it('calls createStore with default middleware', () => {
+      expect(configureStore({ middleware: undefined, reducer })).toBeInstanceOf(
+        Object
+      )
+      expect(redux.applyMiddleware).toHaveBeenCalledWith(
+        expect.any(Function), // thunk
+        expect.any(Function), // immutableCheck
+        expect.any(Function) // serializableCheck
+      )
+      expect(devtools.composeWithDevTools).toHaveBeenCalled()
+      expect(redux.createStore).toHaveBeenCalledWith(
+        reducer,
+        undefined,
+        expect.any(Function)
+      )
+    })
+  })
+
+  describe('given a middleware creation function that returns undefined', () => {
+    it('throws an error', () => {
+      const invalidBuilder = jest.fn(getDefaultMiddleware => undefined as any)
+      expect(() =>
+        configureStore({ middleware: invalidBuilder, reducer })
+      ).toThrow(
+        'when using a middleware builder function, an array of middleware must be returned'
+      )
+    })
+  })
+
+  describe('given a middleware creation function that returns an array with non-functions', () => {
+    it('throws an error', () => {
+      const invalidBuilder = jest.fn(getDefaultMiddleware => [true] as any)
+      expect(() =>
+        configureStore({ middleware: invalidBuilder, reducer })
+      ).toThrow('each middleware provided to configureStore must be a function')
+    })
+  })
+
+  describe('given custom middleware that contains non-functions', () => {
+    it('throws an error', () => {
+      expect(() =>
+        configureStore({ middleware: [true] as any, reducer })
+      ).toThrow('each middleware provided to configureStore must be a function')
+    })
+  })
+
   describe('given custom middleware', () => {
     it('calls createStore with custom middleware and without default middleware', () => {
       const thank: redux.Middleware = (_store) => (next) => (action) =>

src/configureStore.ts

@@ -153,11 +153,26 @@ export function configureStore<
     )
   }
 
-  const middlewareEnhancer = applyMiddleware(
-    ...(typeof middleware === 'function'
-      ? middleware(curriedGetDefaultMiddleware)
-      : middleware)
-  )
+  let finalMiddleware = middleware
+  if (typeof finalMiddleware === 'function') {
+    finalMiddleware = finalMiddleware(curriedGetDefaultMiddleware)
+
+    if (!IS_PRODUCTION && !Array.isArray(finalMiddleware)) {
+      throw new Error(
+        'when using a middleware builder function, an array of middleware must be returned'
+      )
+    }
+  }
+  if (
+    !IS_PRODUCTION &&
+    finalMiddleware.some(item => typeof item !== 'function')
+  ) {
+    throw new Error(
+      'each middleware provided to configureStore must be a function'
+    )
+  }
+
+  const middlewareEnhancer = applyMiddleware(...finalMiddleware)
 
   let finalCompose = compose