formatDate.js

/**
 * Takes a date object and returns a string in the form YYYY-MM-DD
 * e.g. formatDate(newDate()) should return 2017-02-17 for February 17, 2017.
 * 
 * @param {date} d the date object to be formatted
 */
function formatDate (d = new Date()) {
    // explain:
    //       | YYYY format      | add leading 0 to Jan-Sep      | month is 0-indexed  | add leading 0 to days 1-9     | day of month
    return `${d.getFullYear() }-${ d.getMonth() < 9 ? '0' : '' }${ d.getMonth() + 1 }-${ d.getDate() < 10 ? '0' : '' }${ d.getDate()}`;
}