RxJS- How to access the value of a BehaviorSubject after operations

Question

Say I have a behavior subject, piped through an operation or two:

const inputNumber = new BehaviorSubject(2);

// The current number can be retrieved with `inputNumber.getValue()`

const numberAfterOperations = inputNumber.pipe(
  filter(a => (a+1) % 2), // filter out odd numbers
  map(a => a * a), // square it
);

How can I access the value of numberAfterOperations?

// Does not work because `getValue` is not defined!
numberAfterOperations.getValue(); // I expect this to be 4

inputNumber.next(4);

// Does not work!
numberAfterOperations.getValue(); // I expect this to be 16

Note: the observable subscription here works fine. I'm asking about synchronous access to the .value (or .getValue())

I noticed a similar issue here, but did not see a resolution: https://github.com/ReactiveX/rxjs/issues/2378

Answer 1

Thanks for the answers, folks. You have helped me realize that there is no built-in way to do what I wanted with behavior subjects. I wound up with the following:

function mapBehaviorSubject(behavior, mapFn) {
  const mappedSubject = new BehaviorSubject(mapFn(behavior.getValue()));
  behavior.subscribe({
    next: value => mappedSubject.next(mapFn(value)),
  });
  return mappedSubject;
}

This allows me to have the desired behavior:

const inputNumber = new BehaviorSubject(2);

const numberAfterOperations = mapBehaviorSubject(inputNumber, x => x * x);

This allows me to have a persistent subject with the mapped value available synchronously with getValue().