shallow copy vs deep copy

https://medium.com/@nickhuang_1199/%E6%B7%B1%E5%BA%A6%E4%BA%86%E8%A7%A3%E5%9C%A8javascript%E4%B8%AD%E6%B7%BA%E6%8B%B7%E8%B2%9D-shallow-copy-vs-%E6%B7%B1%E5%BA%A6%E6%8B%B7%E8%B2%9D-deep-copy-%E5%BE%97%E9%83%A8%E5%88%86-33fe501871e

類似by value v.s. by reference的概念，專指在複製行為上