【递归思想】 
<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8">
    </head>
    <body>
        
        <script>
            const _flatten = arr => {
                // 补全代码
                let i=0;
           	let resArr = new Array();

           	do{

           		i = 0;
           		
           		while(typeof arr[i] == 'number'){
           			resArr.push(arr[i])
           			i++;
           
           		}
       
           		arr = arr[i];

           		if(arr == undefined){
           			break;
           		}
        


           	}while(typeof arr[0] == 'number' || arr.length == 1)

           	return resArr;
            }
        </script>
    </body>
</html>