Warning: Each child in an array or iterator should have a unique "key" prop.

源代码是这样

class Main extends Component{    render(){        return(            <div>                {                    arr.map((item,index)=>{                        return(                            <li>{item*2}</li>                        )                    })                }            </div>        )    }}render(    <Main />,    document.getElementById("root"));

但会报这样的错误：

bundle.js:1004 Warning: Each child in an array or iterator should have a unique "key" prop. Check the render method of `Main`. See https://fb.me/react-warning-keys for more information.

解决方案如下：

class Main extends Component{    render(){        return(            <div>                {                    arr.map((item,index)=>{                        return(                            <li key={index}>{item*2}</li>                        )                    })                }            </div>        )    }}render(    <Main />,    document.getElementById("root"));