countdown.js

/**
 * @typedef {TimeUnitTuple[]} TimeUnitTable A table of time units (seconds, minutes, for example), with their corresponding multipliers compared to the last value. The order is important, as each tuple contains a `multiplier`, which is how many times the last value would fit into this one (i.e. there are 60 minutes in an hour, so hour would be [60, 'hours'])
 * @typedef {[multiplier: number, label: string]} TimeUnitTuple Intended to be used in an array of the same type, where the `multiplier` represents how many times this unit fits into the last, and `label` is the name of that unit (for example, days)
 */

/**
 * Returns the absolute difference between two date objects
 * @curried
 * @param {(number | Date)} date1
 * @returns {(date2?: number | Date) => number} The difference in milliseconds
 * @example 
 * // returns 1008823000
 * getTimeDifference
 *   (new Date("October 12 2020 16:00:00"))
 *   (new Date("October 24 2020 08:13:43"));
 * @example <caption>The function can take 1 date to get the difference between then and now</caption>
 * // (Assuming that the current date is October 24 2020 08:13:43)
 * // returns 1008823000
 * getTimeDifference(new Date("October 12 2020 16:00:00"))();
 */
const getTimeDifference = (date1) => (date2 = new Date().getTime()) =>
  Math.abs(date2 - date1);

/**
 * Translates an amount of milliseconds into a human readable form
 * @param {number} ms
 * @returns {string} A human readable string, including (if necessary) days, hours, minutes, and seconds
 * @example 
 * // returns '11 days 16 hours 13 minutes 43 seconds'
 * const diff = toHumanReadable(1008823000);
 */
const toHumanReadable = (ms) => {
  /**
   * @type TimeUnitTable
   */
  const unitMap = [
    [1000, "seconds"],
    [60, "minutes"],
    [60, "hours"],
    [24, "days"],
  ];

  /**
   * Loops through unitMap, multiplying the numbers to get the total amount of milliseconds to divide by
   * @param {number} index The index to start at
   */
  const getDivisor = (index) =>
    unitMap.slice(index).reduce((prev, [curr]) => prev * curr, 1);

  /**
   * Loops through unitMap, using the tuples to divide the remaining ms and labeling the step with the appropriate unit
   * @param {number} ms
   * @returns {[string, number]} The text and remaining milliseconds
   */
  const getTimeRemaining = (ms) =>
    unitMap.reverse().reduce(
      ([text, time], [, unitText], i) => {
        const divisor = getDivisor(i);
        const dividend = time / divisor;
        const remainder = time % divisor;
        return dividend > 1
          ? [`${text} ${Math.floor(dividend)} ${unitText}`, remainder]
          : [text, time];
      },
      ["", ms]
    );

  // Let's return only the text
  return getTimeRemaining(ms)[0];
};