浅复制只会复制第一层的元素，嵌套的元素还是原来的引用。

const obj = { a: 1, b: 2 }
const copyObj = Object.assign({}, obj)

const arr = [1, 2, 3]
const copyArr = arr.slice()

深复制会每一层都递归复制

function deepCopy(obj) {
  // 处理简单类型，null 和 undefined
  if (obj == null || typeof obj !== 'object') {
    return obj
  }

  // 处理日期
  if (obj instanceof Date) {
    let copy = new Date()
    copy.setTime(obj.getTime())
    return copy
  }

  // 处理数组
  if (obj instanceof Array) {
    let copy = []
    for (let i = 0; i < obj.length; i++) {
      copy[i] = deepCopy(obj[i])
    }
    return copy
  }

  // 处理函数
  if (obj instanceof Function) {
    const copy = function () {
      return obj.apply(this, arguments)
    }

    return copy
  }

  // 处理对象
  if (obj instanceof Object) {
    let copy = {}

    for (let attr in obj) {
      if (obj.hasOwnProperty(attr)) {
        copy[attr] = deepCopy(obj[attr])
      }
    }

    return copy
  }

  throw new Error('Unable to copy obj as type isn\'t supported ' + obj.constructor.name)
}