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)

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