1. 展开嵌套数组
   检查数组中的项是数组的话也要对其进行展开，将展开的结果通过concat拍平放到当前数组中，完成数组展开

function flat(ar) {let arr = []for (var i = 0; i < ar.length; i++) {if (Array.isArray(ar[i])) arr = arr.concat(flat(ar[i]))else arr.push(ar[i])}return arr
}

2. 实现一个数组的reduce
   reduce的作用是将数组中的值通过某种规律聚合到一起，最终将聚合的结果返回

function reduce(arr, fn, initialValue) {if (arr.length === 0) return initialValuelet hasInitialValue = initialValue !== undefinedlet index, resif (hasInitialValue) {index = 0res = initialValue} else {index = 1res = arr[0]}for (; index < arr.length; index++) {res = fn(res, arr[index], index, arr)}return res
}

3. 实现一个深拷贝
   针对需要拷贝的值再进行一次拷贝

function deepClone(data) {let needCopy = Array.isArray(data) || (data && typeof data === 'object')if (!needCopy) return datalet _data = nullif (Array.isArray(data)) {_data = data.reduce((total, cur) => {total.push(deepClone(cur))return total}, [])} else {_data = Object.keys(data).reduce((total, cur) => {total[cur] = deepClone(data[cur])return total}, {})}return _data
}

4. 实现一个可以循环引用的深拷贝
   如果内容被拷贝过就直接返回，不拷贝了

function deepClone(data) {const caches = [] // 拷贝的内容缓存一下return _deepClone(data)function _deepClone(data) {let needCopy = Array.isArray(data) || (data && typeof data === 'object')if (!needCopy) return dataif (caches.includes(data)) return data // 检查是否被拷贝过caches.push(data)let _data = nullif (Array.isArray(data)) {_data = data.reduce((total, cur) => {total.push(_deepClone(cur))return total}, [])} else {_data = Object.keys(data).reduce((total, cur) => {total[cur] = _deepClone(data[cur])return total}, {})}return _data}
}