function deepClone(obj) {
  if (typeof obj !== 'object' || obj === null) {
    return obj
  }

  let res 

  if (obj instanceof Array) {
    res = []
  } else {
    res = {}
  }

  for (let key in obj) {
    if (obj.hasOwnProperty(key)) {
      res[key] = deepClone(obj[key])
    }
  }

  return res
}

注意：Object.assign()不是深拷貝

語法：Object.assign(target, ...sources)
作用: 將所有可枚舉屬性的值從一個或多個源對象復(fù)制到目標對象。它將返回目標對象。

const target = { a: 1, b: 2 }
const source = { b: 3, c: 4 }

const returnedTarget = Object.assign(target, source)

console.log(target)  // {a: 1, b: 3, c: 4}
console.log(returnedTarget)  // {a: 1, b: 3, c: 4}

// 給 returnedTarget 追加一個屬性
const returnedTarget2 = Object.assign(returnedTarget, { d: 5 })
console.log(returnedTarget2)  // {a: 1, b: 3, c: 4, d: 5}

看似是深拷貝

const souce = { b: 3, c: 4 }
const deepSource = Object.assign({}, source)
console.log(deepSource)  // {b: 3, c: 4}
source.b = 10
console.log(deepSource.b)  // 3

但其實是拷貝的source的第一層級(c:4,d:5)，如果source還有第二層級(d:{...})，就看出不是深拷貝了

const source = { b: 3, c: 4, d: {x:5,y:6}}

const deepObj = Object.assign({}, source)
console.log(deepObj)  
console.log(deepObj.d.x)  // 5
source.d.x = 7
console.log(deepObj.d.x)  // 7