2. let arr = [1, 2, [3, 4, [5, 6]]]
    4. // 循环递归
    5. let flatten1 = function (arr) {
    6.     let ret = []
    8.     for (let i=0; i<arr.length; i++) {
    9.         if (Array.isArray(arr[i])) {
    10.             ret = ret.concat(flatten1(arr[i]))
    11.         } else {
    12.             ret.push(arr[i])
    13.         }
    14.     }
    16.     return ret
    17. }
    19. // console.log(flatten1(arr));
    20. // reduce循环递归
    21. let flatten2 = function (arr) {
    22.     return arr.reduce((result, item) => {
    23.         return result.concat(Array.isArray(item) ? flatten2(item) : item)
    24.     }, [])
    25. }
    27. // console.log(flatten2(arr));
    29. // toString方法
    30. let flatten3 = function (arr) {
    31.     return arr.toString().split(',').map(item => +item)
    32. }
    34. // console.log(flatten3(arr));
    36. // es6扩展运算符
    38. let flatten4 = function (arr) {
    39.     while (arr.some(item => Array.isArray(item))) {
    40.         arr = [].concat(...arr)
    41.     }
    42.     return arr
    43. }
    45. console.log(flatten4(arr));
    47. // es6自带
    48. function f5(arr) {
    49.     return arr.flat(Infinity)
    50. }
    51. // 使用栈
    52. function f7(arr) {
    53.     const stack = [...arr]
    55.     const ret = []
    57.     while(stack.length) {
    58.         const next = stack.pop()
    60.         if(Array.isArray(next)) {
    61.             stack.push(...next)
    62.         } else {
    63.             ret.push(next)
    64.         }
    65.     }
    67.     return ret.reverse();
    68. }
    69. // 解构运算原理，generator
    70. function* flatten(arr) {
    71.     for (const item of arr) {
    72.         if(Array.isArray(item)){
    73.             yield* f6(item)
    74.         } else {
    75.             yield item
    76.         }
    77.     }
    78. }
    80. var arr = [1, 2, [3, 4, [5, 6]]];
    81. const flattened = [...flatten(arr)];