JavaScript function that converts a number into the 2 digits day of the month with leading zeros (01-31). Very useful when working with the output that holds days of the month without leading zeros (1-31).

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') );

Google Closure Library has a great function goog.string.padNumber that extends this gist function with number decimal precision.