无法设置状态数据

请立即尝试: 您需要将此关键字与setState()一起使用

import React, { Component } from 'react';

class App extends Component {
constructor() {
super();
  this.state = {
    resData: [],
  } 
}

componentDidMount() {
fetch(`http://someurl.com/something`)
  .then(function (response) {
    return response.json()
  })
  .then(function (result) {
    this.setState({ resData: result.data })
  })
  .catch(function (error) {
    alert("Username password do not match")
  })
}



render() {
const { resData } = this.state;
return (
  <div>
    {resData &&
      <Header />
      <ErrorBoundary>
        <Content data={resData} />
      </ErrorBoundary>
      <Footer />
        }
  </div>
 );
}

export default App;

Check it now 

警报在setState完成之前运行,请尝试将警报作为对setState的回调运行:

componentDidMount() {
        fetch(`http://someurl.com/something`)
        .then(response => response.json())
        .then(result => this.setState({ resData: result.data }), () => {
          alert(this.state.resData);
        });
    }

试试这个也许会有帮助

     import React, { Component } from 'react';

        class App extends Component {
            constructor() {
                super();

                this.state = { 
                    resData: [], 
                }
            }

            componentDidMount() {
    var that = this;
                fetch(`http://someurl.com/something`)
                .then(response => response.json())
                .then(result => { alert(result.data[0].title); that.setState({ resData: result.data }));

    alert(that.state.resData);
            }

            render() {
var that = this;
                return (
                    <div>
                        <Header />
                        <ErrorBoundary>
                            <Content data={ that.state.resData } />
                        </ErrorBoundary>
                        <Footer />
                    </div>
                );
            }

        export default App;