Get relative date from a known date. Before or after N days.

getRelativeDate.js

#!/usr/bin/env node
/**
 * Use this function like: 
 *   `getRelativeDate("before", 3, new Date("7/2/1992"));`
 *
 * @param {String} beforeOrAfter ("after"|"before")
 * @param {Number} numberOfDays number of days before or after the 'date' param
 * @param {Date} date the 'RelativeToDate' in 'getDateRelativeToDate'
 *
 * @return  * If "before" or "after" is not used as the value of the `beforeOrAfter`
 * param, we return `null`, otherwise we return a Date object
 */
export const getRelativeDate = (beforeOrAfter, numberOfDays, relativeDate) => {
  // If the `beforeOrAfter param/variable is not either "before" or "after", return `null`
  if (!['before', 'after'].includes(beforeOrAfter)) {
    return null;
  }

  // If the consumer did not supply a `relativeDate` param, use current Date
  if (!relativeDate) {
    relativeDate = Date.now();
  }
  // If numberOfDays param not supplied, defalt to 1
  if (!numberOfDays) {
    numberOfDays = 1;
  }

  const d = new Date(relativeDate);
  if (beforeOrAfter === "after") {
    return new Date(d.setDate(d.getDate() + numberOfDays));
  }
  if (beforeOrAfter === "before") {
    return new Date(d.setDate(d.getDate() - numberOfDays));
  }
  return null;
};