How to emit 1's not closely followed by a 2

Question

I have an observable stream which emits numbers, I want another stream that emits all 1's that are not closely followed by a 2 (within say 200ms), so for example from this source stream:

(every character is 100ms)

1...12...112...11121...

The result should be:

..1.......1.....11...1.

How would I do that using rxjs@^6.6.7?

Answer 1

Based on @martin's answer:

when a 1 is emitted,

• we create a new observable that waits for the next element for 200ms
• if no element comes within the 200msec, we emit the 1 and close the subscription
• if an element comes and it's a 2, we emit nothing and close the subscription
• if an element comes and it's a 1, we emit the original value and close the subscription

https://stackblitz.com/edit/rxjs-ihyznt?devtoolsheight=66&file=index.ts

source
  .pipe(
    filter(value => value != 2),
    mergeMap(value =>
      source.pipe(
        first(),
        filter(v => v != 2),
        map(_ => value),
        timeout(200),
        catchError(_ => of(value))
      )
    )
  )
  .subscribe(value => console.log(value));