RxJS get before/after emission values of Observable

I'd like to calculate a difference between emissions of a single Observable.

Is there a way to get both the current and previous emission of an Observable if it emits a new value? I'd like something like this:

ob$.subscribe(val => {
  console.log(val)
  // First emission: [1,2,3]

  // Second emission: [1,2,3,4]

  console.log(arrayDifference(before, after)) // [4]
})

How would I go about that? Do I need to store every emission in an upper-scope variable or is there a fancy RxJS way to do this?

Read more here: https://stackoverflow.com/questions/65709840/rxjs-get-before-after-emission-values-of-observable