<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8">
    </head>
    <body>
        
        <script>
            const _flatten = arr => {
                // 补全代码
                return arr.reduce((prev,curr)=>{
                    if(Array.isArray(curr)){
                        return [...prev,..._flatten(curr)]
                    }else{
                        return [...prev,curr]
                    }
                },[])
            }
        </script>
    </body>
</html>