[RxJS] Implement RxJS `switchMap` by Canceling Inner Subscriptions as Values are Passed Through

switchMap is mergeMap that checks for an "inner" subscription. If the "inner" subscription exists, switchMap unsubscribes from that "inner" subscription which effectively "cancels" any pending pushes.

import { fromEvent, of, Subscriber } from "rxjs"
import {
  scan,
  delay,
  mergeMap,
  switchMap
} from "rxjs/operators"

class MySwitchMapSubscriber extends Subscriber {
  innerSubscription

  constructor(sub, fn) {
    super(sub)

    this.fn = fn
  }

  _next(value) {
    console.log(`outer`, value)
    const o$ = this.fn(value)

    if (this.innerSubscription) {
      this.innerSubscription.unsubscribe()
    }

    this.innerSubscription = o$.subscribe({
      next: value => {
        console.log(`  inner`, value)
        this.destination.next(value)
      }
    })
  }
}

const mySwitchMap = fn => source =>
  source.lift({
    call(sub, source) {
      source.subscribe(
        new MySwitchMapSubscriber(sub, fn)
      )
    }
  })

const observable$ = fromEvent(
  document,
  "click"
).pipe(
  scan(i => i + 1, 0),
  mySwitchMap(value => of(value).pipe(delay(500)))
)

const subscriber = {
  next: value => {
    console.log(value)
  },
  complete: () => {
    console.log("done")
  },
  error: value => {
    console.log(value)
  }
}

observable$.subscribe(subscriber)