react 报错Assign arrow function to a variable before exporting as module default

警告代码：

const defaultState = {
  mykey: 1
}

export default (state=defaultState, action) => {
  let newState = JSON.parse(JSON.stringify(state))
  switch(action.type){
      case "addKeyFn":
          newState.mykey++;
          break;
      default:
          break;
  }
  return newState;
}

这样写会报警告：

修改为：

const reducerState = (state=defaultState, action) =>{
  let newState = JSON.parse(JSON.stringify(state))
  switch(action.type){
      case "addKeyFn":
          newState.mykey++;
          break;
      default:
          break;
  }
  return newState;
}

export default reducerState