session-expire-countdown.js

const { rxObserver } = require('api/v0.3');
const { of, timer } = require('rxjs');
const { delay, expand, takeWhile, map } = require('rxjs/operators');

const sessionDuration = 9;
const warningAmount = 7;

const expireEpoch = Date.now() + sessionDuration
const warningEpoch = expireEpoch - warningAmount;

function getTimeLeft() {
  return expireEpoch - Date.now();
}

function getDelay(timeLeft) {
  return 1;
}

timer(warningEpoch)
  .pipe(
    expand(() => {
      const timeLeft = getTimeLeft();
      return (timeLeft > 0) ?
        of(undefined).pipe(delay(getDelay(timeLeft))) :
        empty();
    }),
    map(() => getTimeLeft()),
    takeWhile(timeLeft => timeLeft > 0),
    map(timeLeft => {
      return timeLeft;
    })

  )
  .subscribe(rxObserver());