Comma Formatted Javascript

I have some javascript that comma-formats number inputs.  Works great unless the number is a negative, and then it is adding a commas between the - sign and the first number.    My changes haven't made a difference.   Any ideas what I need to add?

function formatNumber(num) {
  num = rev(num);
  num = num.replace(/(\d{3})/g, '$1,');
  num = rev(num);
  if(num.charAt(0) == ',') num = num.substr(1);
  return num;
}

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