countdown-javascript/counter.js

export class Countdown {
  constructor(expiredDate, onRender, onComplete) {
      this.onRender = onRender;
      this.onComplete = onComplete;
      this.setExpiredDate(expiredDate);

  }

  setExpiredDate(expiredDate) {
      // get the current time
      const currentTime = new Date().getTime();

      // calculate the remaining time 
      this.timeRemaining = expiredDate.getTime() - currentTime;
      this.timeRemaining <= 0 ?
          this.complete() :
          this.start();
  }


  complete() {
      if (typeof this.onComplete === 'function') {
          this.onComplete();
      }
  }
  static getTime(time) {
      return {
          days: Math.floor(time / 1000 / 60 / 60 / 24),
          hours: Math.floor(time / 1000 / 60 / 60) % 24,
          minutes: Math.floor(time / 1000 / 60) % 60,
          seconds: Math.floor(time / 1000) % 60
      };
  }

  update() {
      if (typeof this.onRender === 'function') {
          this.onRender(Countdown.getTime(this.timeRemaining));
      }
  }

  start() {
      // update the countdown
      this.update();

      //  setup a timer
      const intervalId = setInterval(() => {
          // update the timer  
          this.timeRemaining -= 1000;

          if (this.timeRemaining < 0) {
              // call the callback
              this.complete();

              // clear the interval if expired
              clearInterval(intervalId);
          } else {
              this.update();
          }

      }, 1000);
  }
}