Splits passed credit number into quartets (groups of 4 digits).

splitCardNumber.js

/**
 * Split a credit card into groups of four for more readable display.
 *
 * @param {Integer} number Number to split
 *
 * @return {String} Returns formatted number for display
 */
function splitCardNumber ( number ) {
  return number
    .toString()
    .split( '' )
    .reduce( function ( stack, next, index ) {
      return ( index + 1 ) % 4 ? stack + next : stack + next + ' ';
    }, '' );
}