深度克隆，数组扁平化，快速排序

 1 function deepClone(source) {
 2   if (Array.isArray(source)) {
 3     const target = [];
 4     for (let item of source) {
 5       target.push(deepClone(item));
 6     }
 7     return target;
 8   }
 9   if (Object.prototype.toString.call(source) === "[object Object]") {
10     const target = {};
11     for (let key in source) {
12       target[key] = deepClone(source[key]);
13     }
14     return target;
15   }
16   // 其它类型的克隆
17   return source;
18 }

1 function platten(source) {
2   let target = [];
3   for (let item of source) {
4     Array.isArray(item)
5       ? (target = target.concat(platten(item))) // 向外层数组合并
6       : target.push(item);
7   }
8   return target;
9 }

 1 function fastSort(source) {
 2   let middle = source.shift(),
 3     left = [],
 4     right = [];
 5   for (let item of source) {
 6     // undefined 不做处理
 7     item < middle ? left.push(item) : right.push(item);
 8   }
 9   left = left.length > 1 ? fastSort(left) : left;
10   right = right.length > 1 ? fastSort(right) : right;
11   return [...left, middle, ...right];
12 }