我不确定为什么会收到错误。未处理的拒绝（错误）：给定操作“SET_CARS”，reducer“cars”返回未定义。要忽略操作，您必须明确返回之前的状态。如果您希望此reducer不保存任何值，则可以返回null而不是未定义。

REDUCERS

  import { SET_CARS} from "../actions";
  import { combineReducer } from "redux";

  function cars(state = [], action) {
  switch (action.type) {
  case SET_CARS:
  return 
  action.items;
  default:
  return state; }
  }

 const rootReducer = combineReducers({
 cars
 });

 export default rootReducer;

ACTIONS

 export const SET_CARS = 'SET_CARS';

 export function setCars(items){
  return {
    type: SET_CARS , 
    items
}
}

SearchCars

class SearchCars extends Component {
 constructor() {
  super();


   this.state = {
  tansmition: ""
  };
  }

search() {
let { tansmition } = this.state;
const url = `http://localhost:4000/vehicles/? 
transmission=${tansmition}`;


fetch(url, {
  method: 'GET'
})
.then(response => response.json())
.then(json => {
    this.props.setCars(json.results)
  })
}