在 mapStateToProps 中未定义 Redux 状态

Question

I am currently following this tutorial. I've hit a bit of a snag involving mapStateToProps in the following code:

import React from 'react';
import Voting from './voting';
import {connect} from 'react-redux';

const mapStateToProps = (state) => {
  return {
    pair: state.getIn(['vote','pair']),
    winner: state.get('winner')
  };
}

const VotingContainer = connect(mapStateToProps)(Voting);
export default VotingContainer;

Here is the Voting component that's imported:

import React from 'react';
import Vote from './Vote';
import Winner from './winner';

const Voting = ({pair,vote,hasVoted,winner}) =>
  <div>
    {winner ? <Winner winner={winner}/>  :
      <Vote pair={pair} vote={vote} hasVoted={hasVoted}/>
    }
  </div>

export default Voting;

It is supposed to render two buttons from the pair prop. The vote prop is a function that will be executed on click, hasVoted disables buttons when true and winner only renders the winner component as shown.

The state is expected to be an immutableJS map that looks like this:

Map({
  vote:{
    pair:List.of('Movie A','Movie B')
  }
});

Instead I am getting an error saying that state is undefined in the state.getIn line.

The code setting the state is in index:

const store = createStore(reducer);

const socket = io(document.location.protocol + '//' + document.location.hostname + ':8090');
socket.on('state', state => store.dispatch({
  type: 'SET_STATE',
  state
}));

I have logged store.getState() after setting and it is as expected but undefined in mapStateToProps . I also logged the state variable in above context and it's also as expected.

I also set the state normally and it surprisingly works!:

store.dispatch({
  type: 'SET_STATE',
  state: {
    vote: {
      pair: ['Movie A', 'Movie B']
    }
  }
});

The value of state above is exactly what is received from the server

Lastly here's what my reducer looks like:

import React from 'react';
import {Map, fromJS} from 'immutable';

const reducer = (state = Map(), action) => {
  switch (action.type) {
    case 'SET_STATE':
      return state.merge(action.state);
  }
}

export default reducer;

What am I doing wrong?

EDIT: I realised that mapStateToProps is not being called after the store.dispatch() . I went through the docs for the possible reasons mapStateToProps is not being called and it's not one of them.

Answer 1

You reducer doesn't have a default action in switch statement. Which is why even though you mentioned the initial state in reducer params, undefined is returned as store initial state

import React from 'react';
import {Map,fromJS} from 'immutable';

const reducer = (state = Map() ,action) => {
  switch(action.type){
    case 'SET_STATE': return state.merge(action.state);
    default:
      return state;
  }
}

export default reducer;

Adding the default statement will fix the issue :)

Answer 2

I ended up here too because I had failed to pass my rootreducer function to my createStore method. I had:

const store = createStore(applyMiddleware(...middlewares));

I needed:

const store = createStore(rootReducer(), applyMiddleware(...middlewares));