var arr = [1, [2, [3, 4，5]]];
console.log(flatten(arr)); // [1, 2, 3, 4，5]
方法一：递归实现
var arr = [1, [2, [3, 4, 5]]];function 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;}flatten(arr); // [1, 2, 3, 4，5]
2.reduce
var arr = [1, [2, [3, 4, 5]]];function flatten(arr) { return arr.reduce(function(prev, next){ return prev.concat(Array.isArray(next) ? flatten(next) : next) }, [])}console.log(flatten(arr));// [1, 2, 3, 4，5]
3.扩展运算符
var arr = [1, [2, [3, 4, 5]]];function flatten(arr) { while (arr.some(item => Array.isArray(item))) { arr = [].concat(...arr); } return arr;}console.log(flatten(arr)); // [1, 2, 3, 4，5]
4.split + tostring
var arr = [1, [2, [3, 4, 5]]];function flatten(arr) { return arr.toString().split(',');}console.log(flatten(arr)); // [1, 2, 3, 4，5]
5.正则+json
var arr = [1, [2, [3, 4, 5]]];function flatten(arr) { let str = JSON.stringify(arr); str = str.replace(/([|])/g, ''); str = '[' + str + ']'; return JSON.parse(str); }console.log(flatten(arr)); // [1, 2, 3, 4，5]
6.es6 flat
var arr = [1, [2, [3, 4, 5]]];function flatten(arr) { return arr.flat(Infinity);}console.log(flatten(arr)); // [1, 2, 3, 4，5]
