react:Uncaught TypeError: Cannot read property

伙伴把我的demo重新写了一遍，然后，发现，这个错误。

俺也是找了半天。尼玛属性咋就不存在呢…………

Uncaught TypeError: Cannot read property 'value' of undefined

    at mapStateToProps ((index):42)

    at Connect.configureFinalMapState (connect.js?b19a*:155)

    at Connect.computeStateProps (connect.js?b19a*:142)

    at Connect.updateStatePropsIfNeeded (connect.js?b19a*:204)

    at Connect.render (connect.js?b19a*:340)

    at ReactCompositeComponent.js?19b3****:793

    at measureLifeCyclePerf (ReactCompositeComponent.js?19b3****:74)

    at ReactCompositeComponentWrapper._renderValidatedComponentWithoutOwnerOrContext (ReactCompositeComponent.js?19b3****:792)

    at ReactCompositeComponentWrapper._renderValidatedComponent (ReactCompositeComponent.js?19b3****:819)

    at ReactCompositeComponentWrapper.performInitialMount (ReactCompositeComponent.

这个问题一般都是没有默认返回造成的。比如前面的计算器项目，注释defualt 选项，然后就会复现这个错误

const reducer=(state=states,action)=>{
  switch (action.type){
    case 'clear':
      return{
        ...state,
        value:0
      };
    case 'equal':
      return{
        ...state,
        value:eval(state.value)
      };
    case 'addItem':
      return{
        ...state,
        value:state.value===0?action.text:state.value+action.text
      };
    default:
      return state;
  }
};

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