Rxjs equivalent of scan?

[rickmed]

In reactive systems like rxjs you can have observables that keep state. Would be like a mobx.computed that keeps state returns it's last state. Is it possible in mobx?

[urugator]

Side effects in computeds are generally discouraged, but it's probably fine to mutate non-observable that acts as a local state and is not exposed to anything else:

function scan(reduce, initialValue, keepAlive = false) {
  let prevValue = initialValue;  
  let index = 0;
  const _computed = computed(() => {    
    prevValue = reduce(prevValue, index)
    index++;
    return prevValue;
  }, { keepAlive })
  if (!keepAlive) {
     // Reset if unsubscribed
    onBecomeUnobserved(_computed, () => {
      prevValue = initialValue;
      index = 0;
    })
  }  
  return {
    get() {
      return _computed.get();
    } 
  }
}

Did not test.