[Redux] Expected the reducer to be a function

If you see Error: Expected the reducer to be a function.

message when you call createStore to create store.

Check out the second argument of createStore. It must be a function.

// Error
const store = createStore(rootReducer, composeWithDevTools);

ReactDOM.render(
  <React.StrictMode>
    <Provider store={store}>
      <App />
    </Provider>
  </React.StrictMode>,
  document.getElementById('root'),

// Correct
const store = createStore(rootReducer, composeWithDevTools());

ReactDOM.render(
  <React.StrictMode>
    <Provider store={store}>
      <App />
    </Provider>
  </React.StrictMode>,
  document.getElementById('root'),