Format a given number with thousands delimited by commas

formatNumber

function formatNumber(num) {
  if (!isNaN(Number(num))) {
    var numLength = num.toString().split('').length;
    if (numLength % 3 !== 0) {
      // do complex shit,
      // but return 0 for now
      return 0;
    }
    var regexString = '';
    var formatTemplate = '';
    for (var i = 1; i <= (numLength / 3); ++i) {
      regexString += '(\\d{3})';
      if (i < numLength / 3) {
        formatTemplate += '$' + i.toString() + ',';
      } else {
        formatTemplate += '$' + i.toString();
      }
    }
    var regex = new RegExp(regexString,'g');
    var formattedNumber = num.toString().replace(regex, formatTemplate);
    console.log('regex:', regex);
    console.log('formatTemplate:', formatTemplate);
    return formattedNumber;
  } else {
    return 0;
  }
}