flat()

        深度递归遍历数组，并将所有元素与遍历到的子数组中的元素合并为一个新数组返回

                用法

const newArr = arr.flat(num)

                 

const newArr2 = [1, 2, 3, ['a']].flat()//[1, 2, 3, a]const newArr = [1, 2, 3, ['a', 'b', 'c', ['Aa']]].flat(2)//[1,2,3,"a","b","c","Aa]

flatMap()

        Map()：map 可以遍历数组处理数据，并返回新的数组

        flatMap(): 只能展开一层数组

    const arr = [[1], [2], [3]]let newArr = arr.flatMap(item => [item*3])console.log(newArr); //  [3, 6, 9]let newArr2 = arr.map(item => [item*3])console.log(newArr2); // [[3], [6], [9]]