1. es6 flat

      console.log([1 ,[2, 3]].flat()) // [1, 2, 3]
      console.log([1, [2, [3, [4, 5]]]].flat()) // [1, 2, Array(2)]
     // 指定转换的嵌套层数
      console.log([1, [2, [3, [4, 5]]]].flat(2)) // [1, 2, 3, [4, 5]]
     // 不管嵌套多少层
      console.log([1, [2, [3, [4, 5]]]].flat(Infinity)) // [1, 2, 3, 4, 5]
     // 自动跳过空位
      console.log([1,[2, , 3]].flat()) // [1, 2, 3]

2.es6 ...拓展运算符

      var entries=[1,[2,5],[6,7],9]
      var flat_entries = [].concat(...entries)
      console.log(flat_entries)  //[1, 2, 5, 6, 7, 9]

3.递归实现数组扁平化

     const flatten = arr=>{
        let result = []
        for (let i = 0; i < arr.length; i++) {
          if(Array.isArray(arr[i])){
            result = result.concat(flatten(arr[i]))
          }else{
            result.push(arr[i])
          }
        }
        return result
      }

4.reduce实现

 function fn(arr){
        return arr.reduce((prev,cur)=>{
          return prev.concat(Array.isArray(cur)? fn(cur):cur)
        },[])
      }

5.while实现

// 利用： arr =[]. concat (.…. arr )
 //arr . some ( ele => {
  // return Array . isArray ( ele );
// })

 function myFlat ( arr ){
  while ( arr.some ( Array.isArray ) ){
    arr =[]. concat (… arr )
  }
  return arr ;
}
 let arr2=[1,[2,[3,[6,[7]]1],4,5];
 const newArr2= myFlat (arr2);
 console . log (newArr2);

面试题

   // 要求实现一个函数
      const flatten =(list,level =Infinity)=>{
        // ...
      }
      const array = [1,[2,[3,4,[5]],3]]
      const list1 = flatten(array)
      const list2 = flatten(array,2)

解答：

      const flatten = (list, level = +Infinity) => {
        let res = list
        for (let i = 0; i < level; i++) {
          res = [].concat(...res)
          if (!res.some(item => Array.isArray(item))) break
        }
        return res
      }