Zero pad a number. Defaults to 2 zeros.

pad.js

/**
 * Zero pad a number. Defaults to 2 zeros. Can optionally pad arbitrary
 * lengths and characters.
 *
 * Based on:
 * - "pad a string to get to a determined length?" http://stackoverflow.com/a/14760377/1786459
 * - "Repeat Character N Times" http://stackoverflow.com/a/1877479/1786459
 *
 * @param  {Number}  num
 * @param  {Number}  length
 * @param  {String}  char
 * @return {String}
 */
function pad(num, length, char) {
  length = length || 2;
  var pad = Array(length).join(char || '0');

  return String(pad + num).slice(-1 * length);
}