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RxJS: Managing Operator State
When pipeable operators were introduced in RxJS version 5.5, writing user-land operators became much simpler.
A pipeable operator is a higher-order function: a function that returns another function. And the function that is returned takes an observable and returns an observable. So, to create an operator, you don’t have to subclass Operator and Subscriber. You just write a function.
Simple.
However, there are situations in which you need to take some extra care. In particular, you need to be careful whenever your operator stores internal state.
An example
Let’s look at an example: a debug operator that logs received values and their indices to the console.
Our operator will need to maintain some internal state: the index — which will be incremented each time a next notification is received. A naive approach would be to store that state within the operator function. Like this:
However, this approach has a couple of problems that will effect surprising behaviours and…
