Question

我正在从API读取数据，并且试图使用React在HTML的select标签中显示数据数组中的元素，但是它一直告诉我即使该数据也不是数组好像是在我做console.log时。

render(){
        console.log(this.state.countries.Countries)
        return(
            <div className = "tables-container">
                <h1>Tables</h1>
                <label>Please Select a Country to view data for:  </label>
                <select name = "countries" id = "countries">
                    <option selected = "selected">Select a Country</option>
                    {
                        this.state.countries.Countries.map((data)=>(
                             <option>{data.Country}</option>
                        ))
                    }
                </select>
            </div>
        )
    }

运行此命令时，它显示“ TypeError：无法读取未定义的属性'map'。” console.log结果显示为

(188) [{…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, …]

所以我很确定this.state.countries.Countries应该是一个数组，因此map函数应该可以，但是不能。在我的构造函数中，我有

this.state = {
    countries: [],
}

并且我已经获取了API

    componentDidMount(){
        fetch("https://api.covid19api.com/summary")
            .then((data) => data.json())
            .then((data) => {
                this.setState({
                    countries: data
                })
            })
    }

我已经坚持了一个小时，现在我真的需要帮助弄清楚它。谢谢！

Answer 1

this.state = {
    countries: [],
}

this.state.countries从一个空数组开始。它没有.Countries属性，因此对于您的第一个渲染，this.state.countries.Countries是undefined，并且您无法映射undefined。

在加载数据之前和之后，您需要状态具有相同的形状。因此，如果您想要.countries.Countries嵌套，请将您的初始状态更改为：

this.state = {
  countries: {
    Countries: []
  }
}

如果您只想嵌套一层，请保留初始值并更改以下内容：

// In componentDidMount
this.setState({
  countries: data.Countries
})

// In render
this.state.countries.map(

类型错误，无法读取未定义的属性“地图”

1 个答案: