Uncaught TypeError: Cannot read property 'length' of undefined

来源：14-5 PC端首页模块布局开发（中）

慕的地3571103

2017-11-20

老师您好！我的代码是这样的：

constructor() {

super();

this.state = {

projectNumber: ''

};

}

componentWillMount() {

var myFetchOptions = {

method: 'GET'

};

fetch("。。。。。",myFetchOptions)

.then(response => response.json())

.then(json => this.setState({projectNumber: json}));

};

render(){

console.log(this.state);

console.log(this.state.projectNumber.dataList.length)

然后我从后台获取到的json格式格式是这样的;

{

"errcode": 0,

"dataList": [

{

"id": 1,

"pro_num": "0-不涉及"

{

"id": 2,

"pro_num": "1008"

{

"id": 3,

"pro_num": "1019"

{

"id": 4,

"pro_num": "1079"

{

"id": 5,

"pro_num":

但是想要获取dataList的length会报错？？？

写回答

3回答

鸿鹄yyn

2018-03-12

constructor(){
    super();
    this.state={
      new:'',
    };
}
componentWillMount(){
    console.log("1111");
    var opts = {
        method:"GET"
    }
    fetch("http://newsapi.gugujiankong.com/Handler.ashx?action=getnews&type=top&count=10",opts)
    .then((response) => response.json()) //把response转为json
    .then((responseData) => { // 上面的转好的json
        this.setState({new:responseData})
        //alert(responseData);
        console.log(responseData);
      })
      .catch((error)=> {
          message.success('错误了');
        })
        console.log("1111");
}
render(){

    /*
      '没有加载到任何新闻';*/
var count =this.state.new.length;
// var count=news.length;
console.log(count);
const newsList =count
    ? this.state.new.map((newsItem, index) => (
      <li key={index}>
        <Link to={`details/${newsItem.uniquekey}`} target="_blank">
          {newsItem.title}
        </Link>
      </li>
    ))
    : '没有加载到任何新闻';

    return(
      <div class="topNewsList">
      <Card>
        <ul>
          {newsList}
        </ul>
      </Card>
      </div>
    )
}