将数组降级

例子：var arr = [1,2,3,[5,6],[7,[8,[9]]]] 转换为 [1,2,3,4,5,6,7,8]

es6语法

arr.flat();//展开一级：[1,2,3,5,6,7,[8,[9]]]
arr.flat(2);//展开两级：[1,2,3,5,6,7,8,[9]]
arr.flat(Infinity);//Infinity：全部展开[1,2,3,4,5,6,7,8]

递归实现