雖然function typeof 是function，但 (function () {}) instanceof Object === true

https://stackoverflow.com/questions/35185871/what-is-functions-data-type-function-or-object-in-javascript

https://openhome.cc/Gossip/CodeData/EssentialJavaScript/Prototype.html  new 實例用prototype實現，節省每次建立時再建立個別的function