#JavaScript function that converts a number into the #2-digits day of the month with leading zeros (01-31)

utils.numberToDay.js

/**
 * Turns a number/string to 2 digits day of the month with leading zero.
 * @param {number|string} day to turn into day.
 * @return {string}
 */
function numberToDay(j) {
  return ('0' + j).slice(-2);
}

// Examples:
// Turning different type days into 2 digits number.
console.log( numberToDay(7) );
console.log( numberToDay('21') );

ES new feature - padStart
http://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/padStart

const today = new Date();
const yyyy = today.getFullYear();
const mm = String(today.getMonth() + 1).padStart(2, '0');
const dd = String(today.getDate()).padStart(2, '0');
const fullDate = `${yyyy}/${mm}/${dd}`;