A deep copy means that all of the values of the new variable are copied and disconnected from the original variable. A shallow copy means that certain (sub-)values are still connected to the original variable.
白話：
deep copy：斬斷連結，解開束縛，像刀子切很深一樣deep
shallow copy：難分難捨
參考文章：
How to differentiate between deep and shallow copies in JavaScript