A promised based overservable pattern.

countdown.js

// @ts-check
/**
 * Countdown timer as an observable.
 *
 * @param {number} timeout - Specified time to resolve the countdown specified in seconds.
 * @param {?(number) => void} callback - Callback that updates on countdown change.
 * @returns {{ cancel?: (string) => void, promise: Promise<void>}}
 */
export function countdown(timeout, callback = null) {
  const ret = {};
  const signal = new Promise((res, rej) => {
    ret.cancel = (err) => {
      rej(new Error(err));
    }
  });

  ret.promise = new Promise((res, rej) => {
    let index = timeout;
    const interval = setInterval(() => {
      index--
      if (!!callback) {
        callback(index);
      }
      if (index <= 0) {
        res();
        clearInterval(interval);
      }
    }, 1000);

    signal.catch(err => {
      rej(err);
      clearInterval(interval);
    })
  });

  return ret;
}