问题描述
*****我的代码*****
import logo from './logo.svg';
import './App.css';
import React,{ Component } from 'react';
import axios from 'axios';
export default class App extends Component {
state = {
arraydata: []
};
componentDidMount = () => {
this.getmongodbdata(); }
getmongodbdata = () => {
axios.get('http://localhost:8080/fetch').then(res => {
console.log(res.data,)
console.log('Data is recieved')
this.setState({arraydata: res.data})
})
.catch(() => {
alert('Sorry,Could not able to fetch the data');
}) }
render() {
return (
<div className="Main">
<header className="App-header">
<img src={logo} className="App-logo" alt="logo" />
<h1 className="App-title">Welcome to Future</h1>
</header>
<h1> hello world</h1>
<ul key={'qw'}>
{this.state.arraydata.map((data,index) => {
return <li key={`ui-${index}`}>
{data._id}
</li>
})}
</ul>
</div>
) } }
目前 我通过以下方式从数据中获取 ID:
<ul key={'qw'}>
{this.state.arraydata.map((data,index) => {
return <li key={`ui-${index}`}>
{data._id}
</li>
})}
</ul>
***我想得到什么***
是进入data.site.alerts.alert
(显示警报列)
为此我正在努力
<ul key={'qw'}>
{this.state.arraydata.map((data,index) => {
return <li key={`ui-${index}`}>
{data._id}
<ul key={`alert-ui-${index}`}>
{data.site.alerts.map((alertData,alertIndex)=>{
return <li key={`alert-li-${alertIndex}`}>{alertData.alert}</li>
});
</ul>
</li>
})}
</ul>
解决方法
我经常发现即使映射的数据不是未定义的,react 也会抛出错误,所以很多时候你只需要使用一些内联条件,比如 @prasanth 说:
<ul key={'qw'}>
{this.state.arraydata?.map((data,index) => {
return <li key={`ui-${index}`}>
{data._id}
<ul key={`alert-ui-${index}`}>
{data.site.alerts.map((alertData,alertIndex)=>{
return <li key={`alert-li-${alertIndex}`}>{alertData.alert}</li>
});
</ul>
</li>
})}
</ul>
或
<ul key={'qw'}>
{this.state.arraydata ? this.state.arraydata.map((data,alertIndex)=>{
return <li key={`alert-li-${alertIndex}`}>{alertData.alert}</li>
}) : /* say something */;
</ul>
</li>
})}
</ul>
或类似的东西。