React .map() is not a function error

问题

import React, {Component} from 'react';
import './App.css';
import axios from 'axios';

export default class App extends Component {

    constructor() {
        super();
        this.state = {
            weather: []
        };
    }
componentDidMount (){

  this.search();
}
    search = () => {

        axios.get("http://api.openweathermap.org/data/2.5/weather?q=Houston&APPID=f2327cca8f3d665f4c9f73b615b294ed")
        .then(res => {

            this.setState({weather: res.data});

        }).catch(error => {
            console.log('Error from fetching data', error);
        })
    }

    render() {
    console.log(this.state.weather);
    const weathermap = this.state.weather.map( (maps) =>{
      {maps.wind.speed}
});

        return (
            <div className="container">
                <div className="row">
                    <div className="col-md-4 col-md-offset-4">
                        <div className="weather">
                            <div className="current">
                                <div className="info">
                                    <div>&nbsp;</div>
                                    <div className="city">
                                        <small>
                                            <small>CITY:</small>
                                        </small>
                                        {this.state.weather.name}</div>
                                    <div className="temp">67&deg;
                                        <small>F</small>
                                    </div>
                                    <div className="wind">
                                        <small>
                                            <small>WIND:{weathermap}</small>
                                        </small>

                                        </div>
                                    <div>&nbsp;</div>
                                </div>
                                <div className="icon">
                                    <span className="wi-day-sunny"></span>
                                </div>
                            </div>
                            <div className="future">
                                <div className="day">
                                    <h3>Mon</h3>
                                    <p>
                                        <span className="wi-day-cloudy"></span>
                                    </p>
                                </div>
                                <div className="day">
                                    <h3>Tue</h3>
                                    <p>
                                        <span className="wi-showers"></span>
                                    </p>
                                </div>
                                <div className="day">
                                    <h3>Wed</h3>
                                    <p>
                                        <span className="wi-rain"></span>
                                    </p>
                                </div>
                            </div>
                        </div>
                    </div>
                </div>
            </div>

        );
    }
}

I am getting a "this.state.weather.map is not a function" error. I am fetching data from weather api. I got the name from api displayed ok. api call it self is ok is success too. here how the api looks like in console

here is the code

回答1:

You are instantiating the app by saying this.state = { weather: []};. However, When you say this.setState({weather: res.data}), you are overriding the this.state.weather to a JavaScript object rather than array, thus the .map is not available anymore.

You can achieve what you're trying to do by simply const weathermap = this.state.weather.wind.speed

回答2:

this.state.weather is a javascript object so .map is not available you can use

Object.Keys(YourObject).map() 

回答3:

You should do conditional check and then do .map like

.map with return syntax

const { weather } = this.state;
const weathermap = weather && weather.map(maps => {
      return <span>{maps.wind && maps.wind.speed}</span>
});

.map without return syntax

const { weather } = this.state;
const weathermap = weather && weather.map(maps => (
     <span>{maps.wind && maps.wind.speed}</span>
));

来源：https://stackoverflow.com/questions/42989716/react-map-is-not-a-function-error