fretboards | Display fretboards on a browser using Javascript

Manually changing the DOM in React components is an example of side effect and therefore should be put inside useEffect

Try:

When using redux-observable without any other side effect middleware (like redux-thunk) means all actions you dispatch must be plain old JavaScript objects--that includes anything your epics emits.

It's not clear what updateFretboard() returns, except that is probably isn't a POJO action; it's whatever fretboards.child(firebaseKey).update(fretboardData) returns.

If instead of emitting an action, you actually meant to just perform that as a side effect but ignore its return value entirely, you would use something like the do() operator, which is used to make a side effect without actually modifying the next'd values. You could then combine that with the ignoreElements() operator to prevent your epic from emitting anything ever.