尝试渲染从后端获取的项目列表，但出现此错误，表明未定义。但是，当我检查控制台日志时，我可以看到组件的状态数组中肯定有 5 个项目。

class PubSubTopics extends React.Component{
    constructor(props){
        super(props);
        this.state = {
            pubsubtopics: ['one', 'two', 'three'],
        }
    }

    componentDidMount() {
        this.callBackEndAPI()
            .then(res =>
                this.setState({pubsubtopics: res.express}))
            .catch(err => console.log(err));
        console.log('after setting state');
        console.log(this.state.pubsubtopics);
    }


    callBackEndAPI = async () => {
        const response = await fetch('/listtopics');
        const body = await response.json();

        if(response.status !== 200){
            throw Error(body.message)
        }
        console.log('after API responded');
        console.log(body.topics);
        return body.topics;
    }
    render(){
        const list = [];
        for(const[index, value] of this.state.pubsubtopics){
            list.push(<li key={index}>{value}</li>)
        }
        return(
            <div>
                <ul>
                    {list}
                </ul>
                <button onDoubleClick={this.handleClick}/>
            </div>
        )
    }
}

控制台日志：

after setting state
index.js:21 (3) ["one", "two", "three"]
index.js:32 after API responded
index.js:33 (5) [{…}, {…}, {…}, {…}, {…}]

知道为什么它说 this.state.pubsubtopics 未定义吗？