componentDidUpdate.js

componentDidUpdate(prevProps) {
  const clockPropsHasChanged = ['hours', 'mins', 'sec', 'countdown'].some(
    prop => this.props[prop] !== prevProps[prop]
  );
  if (clockPropsHasChanged) {
    clearInterval(this._interval);
    let { countdown, hours, mins, sec } = this.props;
    if (countdown) {
      this._interval = setInterval(() => {
        sec -= 1;
        if (sec < 0) {
          sec = 59;
          mins -= 1;
        }
        if (mins < 0) {
          mins = 59;
          hours -= 1;
        }
        if (hours < 0) {
          clearInterval(this._interval);
        } else {
          this.setState({ hours, mins, sec });
        }
      }, 1000);
    }
  }
}