Redux: Call to action-creator returns 'Cannot read property 'props' of undefined'

问题

I'm able to fetch the weather by a given city but can't figure how to delete it afterwards. Being still new to Redux I'm trying to figure which pieces am I missing on this puzzle.

I'm leaving the FETCH_WEATHER code in just for context proposes. My issues is with the DELETE_CITY implementation.

Bug:

Steps:

Add a button in each row that calls an action creator.
Action creator takes the name of the 'city' and return an action of type 'DELETE_CITY'.
Reducer, watches for that type and iterates through the array of cities and deletes the object with the given city name.

action creator

import axios from 'axios';
const API_KEY = '1111111111111111111111';
const ROOT_URL = `http://api.openweathermap.org/data/2.5/forecast?appid=${API_KEY}`;

export const FETCH_WEATHER = 'FETCH_WEATHER';
export const DELETE_CITY = 'DELETE_CITY';

//fetch cities
export function fetchWeather(city){
    const url = `${ROOT_URL}&q=${city},&units=imperial`;
    const request = axios.get(url);
    console.log('Request:', request);

    return {
        type: FETCH_WEATHER,
        payload: request
    }
}

//Delete Cities
export function deleteCity(city) {
    return {
        type: DELETE_CITY,
        payload: city
    }
}

Reducer: index.js

import { combineReducers } from 'redux';
import WeatherReducer from './reducer_weather';

const rootReducer = combineReducers({
  weather: WeatherReducer
});

export default rootReducer;

weather_reducer.js

Is the case for DELETE_CITY implemented accurately?

import { FETCH_WEATHER } from '../actions/index';
import { DELETE_CITY } from '../actions/index';

export default function (state = [], action) {
    console.log('Action received', action);

    //only fetch weather data type
    switch (action.type) {
        case FETCH_WEATHER:
            //concat to prevent state mutation
            return state.concat([action.payload.data]);

        case DELETE_CITY:
            return state
                .slice(0, action.payload.data)
                .concat([action.payload.data].slice([action.payload.data] + 1));
    }
    return state;
}

Add a button in each row that calls an action creator.

Ps:Are mapStateToProps and mapDispatchToProps correct?

weather_container.js

import React, { Component } from 'react';
import { connect } from 'react-redux';
import Chart from '../components/chart';
import GoogleMap from '../components/google_map';
import { bindActionCreators } from 'redux';
import { deleteCity } from '../actions/index';

class WeatherList extends Component {

    renderWeather(cityData) {
        const name = cityData.city.name;

        return (
            <tr key={name}>
                <td><button onClick={() => this.props.deleteCity(cityData)} className="btn btn-danger">x</button></td>
            </tr>
        )
    }
    render() {
        return (
            <table>
                <thead>
                    <tr>
                        <th>City</th>
                    </tr>
                </thead>
                <tbody>
                {this.props.weather.map(this.renderWeather)}
                </tbody>
            </table>
        )
    }
}

 function mapStateToProps(state) {
     return {
         weather: state.weather
     }
 }

 function mapDispatchToProps(dispatch, weather) {
     return bindActionCreators({deleteCity},{weather}, dispatch)
 }

 export default connect(mapStateToProps, mapDispatchToProps)(WeatherList);

I need to bind the action creator to the onClick event. I tried both methods: fat arrow functions and/or 'binding the method inside of a constructor()'.

They both return with props undefined.

回答1:

Try replaceing:

{this.props.weather.map(this.renderWeather)}

With

{this.props.weather.map(this.renderWeather, this)}

The second argument of map is the thisArg - value to use as this when executing callback.

回答2:

You need to bind renderWeather function.

class WeatherList extends Component {
    constructor(){
        super();
        this.renderWeather = this.renderWeather.bind(this);
    }
    ...
}

回答3:

The error message you're getting cannot read property props of undefined means that in this.props this is undefined.

It's a common gotcha in react, mainly because this is falling out of context.

What you need to do is bind the renderWeather function to WeatherList in the constructor

constructor() {
   this.renderWeather = this.renderWeather.bind(this)
}

Another alternative is to use arrow functions

class {
   renderWeather = (weatherData) => {
       ...
   }
   ...
}

You might also consider this implementation in you reducer when deleting a city.

[...state].filter((weather) => {
    return weather.id != action.payload.data.id
})

.filter() returns a new array of items that meet the given condition

来源：https://stackoverflow.com/questions/45286460/redux-call-to-action-creator-returns-cannot-read-property-props-of-undefined

标签

javascript

reactjs

redux

react-redux