ReactJS：类型错误：this.state.data.map 不是函数

-有人可以帮我解决这个问题吗？我收到此问题标题中描述的错误。我想将 JSON 数据放入react-grid-layout 组件中。该库可以在（https://github.com/STRML/react-grid-layout https://github.com/STRML/react-grid-layout)

import React, {Component} from 'react';
import ReactDOM from 'react-dom';
import Employee from './Employee'; 
import "./react-grid-layout/css/styles.css";
import "./react-resizable/css/styles.css"


var ReactGridLayout = require('react-grid-layout');

var data = [];

class MyFirstGrid extends React.Component {
    constructor() {
        super();
        this.state = {
            data: []
        };
    }

    componentDidMount(){
        fetch("http://localhost:8080/TrainingWebMvcSpring2/roottypes/listAll.do")
            .then( (response) => {
                return response.json() })
                    .then( (json) => {
                        this.setState({data: json});
                    });
    }


    render () {
        console.log(data);

        return (
            <ReactGridLayout className="layout" cols={12} rowHeight={30} width={1200}>
                {this.state.data.map(function(item, key){
                    return(
                        <div>
                        <div key="a" data-grid={{x: 0, y: 0, w: 1, h: 2, static: true}}> {item}</div>
                        <div key="b" data-grid={{x: 1, y: 0, w: 3, h: 2, minW: 2, maxW: 4}}> {item} </div>
                        <div key="c" data-grid={{x: 4, y: 0, w: 1, h: 2}}> {item} </div>
                        </div>
                    )
                })}
            </ReactGridLayout>
        )
    }
}

export default MyFirstGrid;

ReactJS：TypeError：this.state.data.map 不是一个函数？

Array#map https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map在数组上可用。

调用 fetch API 后，

this.setState({data: json});

json作为对象返回，而不是转换的数组data到一个物体

将其更改为：

this.setState({data: [...json]});