面试题：多维数组降维的几种方法（必会）

（1）数组字符串化：

 // （1）数组字符串化：
      let arr = [
        [222, 333, 444],
        [55, 66, 77]
      ]
      arr += ''
      arr = arr.split(',')
      console.log(arr) // ["222", "333", "444", "55", "66", "77"]

（2）递归

      // （2）递归
      function reduceDimension(arr) {
        let ret = []
        let toArr = function (arr) {
          arr.forEach(function (item) {
            item instanceof Array ? toArr(item) : ret.push(item)
          })
        }
        toArr(arr)
        return ret
      }
      let arr = [
        [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
        [11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
      ]
      console.log(reduceDimension(arr))

（3）flat()

 // （3）flat()
      let arr = [ // 二维数组
        [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
        [11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
      ]
      console.log(arr.flat()) // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]

（4）flatMap()

      // （4）flatMap()
      let arr = [ [1,2,3], [4,5,6], [7,8,9] ]
#### console.log(arr.flatMap(item => item)) // [1, 2, 3, 4, 5, 6, 7, 8, 9]