This chapter has covered functional programming and asynchronous data streams. Working with RxJS doesn't require a deep understanding of functional programming, but you do need to understand what declarative means, in order to focus on the right things. Your focus should be on what you want done, not how you want it done. RxJS, as a library, will take care of the how. More on that in the upcoming chapter.

These might seem like two different topics. Combining the two, however, gives us the ability to manipulate streams. A stream can be seen as a list of data, where the data is available at a certain point in time. If we start treating our streams as lists, especially immutable lists, then there are operations that go with lists that manipulate lists by applying operators to them. The result of the manipulation is a new list, not a mutated list. So let's start applying our list philosophy and its operators to the following situations.