Native JavaScript Countdown script (JQuery required. Will be updated very soon)

countdown.js

 /**
  * John Cyrill Corsanes
  * https://jsfiddle.net/jccultima123/L4tdpuc6/
  *
  * NOTE: Manually set this function with intervals on target pages
  * to avoid issues came from turbolinks
  *
  * SAMPLE:
  * ```
  * $(function() {
  *   setInterval(() => {
  *     countDown('#dateTimeCountdown', $('#activePromoEnd').val());
  *   }, 1000);
  * });
  * ```
  * TODO: Native element selector
  *
  * @param {String} elementStr -- Should be `span` or `p`
  * @param {String} endDateTime
  */
function countDown(elementStr, endDateTime = null) {
  // params
  var startDateTime = new Date();
  var endDateTime = new Date(endDateTime);
  // Time remaining (difference)
  var timeRemaining = (endDateTime - startDateTime);
  // Update if has difference, otherwise hide
  if (timeRemaining > 0) {
    // Formula
    var a_day = 24 * 60 * 60 * 1000;
    // Compute by days
    var d = Math.floor(timeRemaining / a_day)
    // Compute by hours
    var h = Math.floor((timeRemaining % a_day) / (60 * 60 * 1000))
    // Compute by minutes
    var m = Math.floor((timeRemaining % a_day) / (60 * 1000)) % 60
    // Compute by seconds
    var s = Math.floor((timeRemaining % a_day) / 1000) % 60 % 60
    // Put text
    var result = d + ' days, ' + h + ' hours, ' + m + ' minutes, ' + s + ' seconds'
    $(elementStr).text(result).parent().removeClass("hide").show();
  } else {
    // Hide container
    $(elementStr).parent().hide();
  }
}