Format a number with thousand's place seperating commas.

add_commas.js

/**
 * Format a number with thousand's place seperating commas.
 * @param {number} num is the number to format.
 * @returns {string} The formatted number.
 */
function addCommas( num ) {
  var numString = num + '';
  var numArray = numString.split('').reverse();

  if ( numArray.length < 4 ) {
    return numString;
  }

  for ( let i = 1, length = numArray.length; i < length; i++ ) {
    if ( i % 3 === 0 ) {
      numArray[i] += ',';
    }
  }

  return numArray.reverse().join('');
}