function number_with_commas(n){
    var m = Math.abs(n);
    var sign = n < 0 ? '-' : '';
    var output = "";
    var chars = String(m).split('').reverse();
    var commas = Math.floor((chars.length-1) / 3);
    var i = 0;
    for(i=0; i<commas; i++){
      output = output + chars.slice(0,3).join('') + ',';
      chars = chars.slice(3);
    }
    output = output + chars.join('');
    return sign + output.split('').reverse().join('');
  }

A quick function that makes it easy to convert an integer into a number string with commas inserted.

Add commas to numbers

number, format

JavaScript