Pipeable Operators are the kind that can be piped to Observables using the syntax observableInstance.pipe(operator()). These include, filter(…), and mergeMap(…). When called, they do not change the existing Observable instance. Instead, they return a new Observable, whose subscription logic is based on the first Observable.
A Pipeable Operator is a function that takes an Observable as its input and returns another Observable. It is a pure operation: the previous Observable stays unmodified.
operator就是函數式程式設計世界裡的一等公民，接收一個Observable作為輸入，傳回另一個新的Observable對象。
Operators are the essential pieces that allow complex asynchronous code to be easily composed in a declarative manner.
借助operator，我們可以使用一種聲明式的方式來進
