1. function deepClone(obj, hash = new WeakMap()) {
    2.   if (obj === null) return obj
    3.   if (obj instanceof Date) return new Date(obj)
    4.   if (obj instanceof RegExp) return new RegExp(obj)
    5.   if (typeof obj !== 'object') return obj
    6.   if (hash.get(obj)) return hash.get(obj)
    7.   let newObj = new obj.constructor()
    8.   hash.set(obj, newObj)
    9.   for (let key of Object.keys(obj)) {
    10.     newObj[key] = deepClone(obj[key], hash)
    11.   }
    12.   return newObj
    13. }