Implementation using "takeUntil".ts

// Original code
const pingEpic = (
  action$,
) => (
  action$
  .pipe(
    ofType(PING),
    delay(1000),
    map(pong),
  )
)

// Accepted solution.
const pingEpic = (
  action$,
) => (
  action$
  .pipe(
    ofType(PING),
    switchMap(() => (
      timer(1000)
    )),
    map(pong),
  )
)

// This kills the entire observable when another PING comes in. Not what you want.
const pingEpic = (
  action$,
) => (
  action$
  .pipe(
    ofType(PING),
    takeUntil(
      action$
      .pipe(
        ofType(PING)
      )
    ),
    delay(1000),
    map(pong),
  )
)

// Instead of using `switchMap`, you could use `takeUntil` and `mergeMap` just fine.
const pingEpic = (
  action$,
) => (
  action$
  .pipe(
    ofType(PING),
    mergeMap(() => (
      timer(1000)
      .pipe(
        takeUntil(
          action$
          .pipe(
            ofType(PING)
          )
        )
      )
    )),
    map(pong),
  )
)

// I use namespaces often for reusable epics and reducers (https://itnext.io/the-secret-to-using-redux-createnamespacereducer-d3fed2ccca4a)
// They comes in handy when you're using one epic for multiple uses.
const pingEpic = (
  action$,
) => (
  action$
  .pipe(
    ofType(CREATE_PING_LISTENER)
    mergeMap(({
      namespace,
    }) => (
      action$
      .pipe(
        ofType(PING),
        ofNamespace(namespace),
        takeUntil(
          action$
          .pipe(
            ofType(STOP_PING_LISTENER)
          )
        ),
        switchMap(() => (
          timer(1000)
        )),
        mapTo({ namespace }),
        map(pong),
    )),
  )
)