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

干就完事了