如何解决ReactJS：未捕获的ReferenceError：[变量]未定义
我是Reactjs的新手，但我有一个未定义变量的问题。在控制台日志中，我从以下React页面获取“ Uncaught ReferenceError：未定义drawResults app.js：116002”：
import React,{ Component } from "react";
import ReactDOM from "react-dom";
import { render } from 'react-dom';
import '@fortawesome/fontawesome-free/css/all.min.css';
import "bootstrap-css-only/css/bootstrap.min.css";
import "mdbreact/dist/css/mdb.css";
import { NumberPicker } from 'react-widgets';
import NumberPicker from 'react-widgets/lib/NumberPicker'
import PropTypes from "prop-types";
import Radium,{Style} from 'radium';
import { MDBBtn,MDBInput,MDBModal,MDBModalBody,MDBModalHeader,MDBModalFooter,MDBIcon,MDBBadge,MDBContainer,MDbrow,MDBCol,MDBSelect,MDBInputNumeric} from "mdbreact";
import { event } from "jquery";
import axios from 'axios';
class App extends Component {
static propTypes = {
kind: PropTypes.oneOf(['input','button'])
};
constructor(props) {
super(props);
this.state = {
drawResults: [],mainDrawSet: 40,mainBallsDrawn: 5,powerBallSet: 5,powerballBallsDrawn: 0
};
}
componentDidMount(){
this.fetchResults();
}
playLotto (mainDrawSet,mainBallsDrawn,powerBallSet,powerballBallsDrawn) {
fetch('/draw',{
method: 'post',body: JSON.stringify({draw:this.state}),headers: {
'Access-Control-Allow-Origin':'*','Content-Type':'x-www-form-urlencoded','Accept': 'application/json','Content-Type': 'application/json;charset=UTF-8',}
})
.then(res => {
console.log(res)
})
.catch(err => {
console.log(err)
})
console.log(this.state);
};
submit () {
this.playLotto(this.state.mainDrawSet,this.state.mainBallsDrawn,this.state.powerBallSet,this.state.powerballBallsDrawn)
this.setState({
mainDrawSet: 40,powerballBallsDrawn: 0
})
};
exportTasks = (_this) => {
let _url = $(_this).data('href');
window.location.href = _url;
}
fetchResults = async () => {
try {
const res = await fetch('/lotto_results',{
method: 'GET',headers: {
'Accept': 'application/json','Content-Type': 'application/json'
}
});
const res_1 = await res.json();
return this.setState({ drawResults: res_1.data });
} catch (err) {
return console.log(err);
}
}
render() {
return (
<MDBContainer>
<MDbrow>
<MDBCol md="12" className="mb-r">
<h2 className="text-uppercase my-3">Lotto Draw Machine:</h2>
<div data-test="container" className="container-fluid section mb-5 border p-3">
<form className="mx-3 grey-text row" style={{ position: "relative",marginTop: "1.5rem",marginBottom:"1.5rem"}} data-test="row">
<div className="form-row align-items-center">
<div data-test="col" className="col">
<div className="md-form form-group form-lg md-bg" style={{position: "relative",marginBottom: "1.5rem"}}>
<label style={{ position: "relative",marginBottom:"1rem"}} htmlFor="mainDrawSet">Number of balls in main draw set</label>
<NumberPicker min={40} max={49} defaultValue={40} type="number" className="custom-select" id="mainDrawSet" value={this.state.mainDrawSet} onChange={mainDrawSet => this.setState({ mainDrawSet})} style={{width: "400px",height: "40px"}}/>
<label style={{ position: "relative",marginBottom:"1rem"}} htmlFor="mainDrawSet">Please choose a number between 40 and 49</label>
</div>
</div>
<div data-test="col" className="col">
<div className="md-form form-group form-lg md-bg">
<label style={{ position: "relative",marginBottom:"1rem"}} htmlFor="mainDraw">Number of balls in main draw</label>
<NumberPicker min={5} max={7} defaultValue={5} type="number" className="custom-select" id="mainDraw" value={this.state.mainBallsDrawn} onChange={mainBallsDrawn => this.setState({ mainBallsDrawn })} style={{width: "400px",height: "40px"}}/>
<label style={{ position: "relative",marginBottom:"1rem"}} htmlFor="mainDraw">Please choose a number between 5 and 7</label>
</div>
</div>
</div>
<div className="mx-3 grey-text row" style={{ position: "relative",marginBottom:"1.5rem"}} data-test="row">
<div className="form-row align-items-center">
<div data-test="col" className="col">
<div className="md-form form-group form-lg md-bg" style={{position: "relative",marginBottom: "1.5rem"}}>
<label htmlFor="powerBallSet" style={{ position: "relative",marginTop: "0.5rem",marginBottom:"0.5rem"}}>Powerball Set</label>
<NumberPicker min={5} max={49} defaultValue={5} type="number" className="custom-select" id="powerBallSet" value={this.state.powerBallSet} onChange={powerBallSet => this.setState({ powerBallSet })} style={{width: "400px",height: "40px"}}/>
<label htmlFor="powerBallSet" style={{ position: "relative",marginBottom:"0.5rem"}}>Please choose a number between 5 and 49</label>
</div>
</div>
<div data-test="col" className="col">
<div className="md-form form-group form-lg md-bg">
<label htmlFor="powerBall" style={{ position: "relative",marginBottom:"0.5rem"}}>Number of powerball balls</label>
<NumberPicker min={0} max={3} defaultValue={0} type="number" className="custom-select" value={this.state.powerballBallsDrawn} onChange={powerballBallsDrawn => this.setState({ powerballBallsDrawn})} id="powerBall" style={{width: "400px",height: "40px"}}/>
<label htmlFor="powerBall" style={{ position: "relative",marginBottom:"0.5rem"}}>Please choose a number between 0 and 3</label>
</div>
</div>
</div>
</div>
<div className="mx-3 grey-text row" style={{ position: "relative",marginBottom:"1.5rem"}} data-test="row">
<div data-test="row" className="row">
<button data-test="button" type="button" className="btn-info btn Ripple-parent" style={{width: "400px",height: "40px"}} onClick={()=>{this.submit()}}>Let's Play!<div data-test="waves" className="Ripple "></div></button>
</div>
</div>
</form>
</div>
</MDBCol>
</MDbrow>
<MDbrow>
<MDBCol md="7" className="mb-r">
<div className="mx-3 grey-text row" style={{ position: "relative",marginBottom: "1.5rem"}}>
<h4 className="text-uppercase my-3">Lotto Results:</h4>
</div>
</div>
<div data-test="col" className="col">
<div className="md-form form-group form-lg md-bg">
<button data-href="/tasks" data-test="button" type="button" className="btn-info btn mr-5 Ripple-parent" style={{width: "400px",height: "40px"}} onClick={()=>{this.exportTasks(event.target)}}>Export To CSV</button>
</div>
</div>
</div>
</div>
</MDBCol>
</MDbrow>
<MDbrow>
<MDBCol md="12" className="mb-r">
return {drawResults.map((result,index) =>
<div key={index} className="mx-3 grey-text row" style={{ position: "relative",marginBottom:"1.5rem"}} data-test="row">
<table className="table center">
<thead className="grey lighten-2">
<tr>
<th scope="col">Draw Time</th>
<th scope="col"> Draw Result</th>
<th scope="col"> Number of Main Balls Drawn</th>
<th scope="col">Number of Powerball Balls Drawn</th>
</tr>
</thead>
<tbody>
<tr>
<th scope="row">result.draw_time</th>
<td>result.results</td>
<td>result.main_draw_balls</td>
<td>result.power_balls</td>
</tr>
</tbody>
</table>
</div>
)}
</MDBCol>
</MDbrow>
</MDBContainer>
);
}
}
App = Radium(App);
export default App;
if (document.getElementById('app')) {
ReactDOM.render(
<App />,document.getElementById('app'));
}
可能有人建议如何解决该错误。我已经在构造函数中声明了状态变量，并尝试在fetchResults（）方法中为其分配值，但是脚本失败，变量未定义。
解决方法
我认为您的问题来自 return this.setState({ drawResults: res_1.data });
由于setState只是修改了组件的状态，因此您无法返回setState，您应该尝试使用this.setState({ drawResults: res_1.data });
该脚本失败，并显示“未定义”错误，因为该变量确实未定义。
您说您已经在构造函数中声明了它。您实际上在构造函数中执行的操作是创建this.state的属性（或更准确地说，创建您随后分配给this.state的对象的属性）。很好，但是您需要引用属性，而不是变量。
要解决该错误，请将drawResults更改为this.state.drawResults。
编辑：我打算更改此行：return {drawResults.map(...
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