Works on all browsers, this is all you need.

  function commaSeparateNumber(val){

    while (/(d+)(d{3})/.test(val.toString())){

      val = val.toString().replace(/(d+)(d{3})/, '$1'+','+'$2');

    }

    return val;

  }

Wrote this to be compact, and to the point, thanks to regex. This is straight JS, but you can use it in your jQuery like so:

$('#elementID').html(commaSeparateNumber(1234567890));

or

$('#inputID').val(commaSeparateNumber(1234567890));

However, if you require something cleaner, with flexibility. The below code will fix decimals correctly, remove leading zeros, and can be used limitlessly. Thanks to @baacke in the comments.

  function commaSeparateNumber(val){

   val = val.toString().replace(/,/g, ''); //remove existing commas first

   var valRZ = val.replace(/^0+/, ''); //remove leading zeros, optional

   var valSplit = valRZ.split('.'); //then separate decimals

    

   while (/(d+)(d{3})/.test(valSplit[0].toString())){

    valSplit[0] = valSplit[0].toString().replace(/(d+)(d{3})/, '$1'+','+'$2');

   }



   if(valSplit.length == 2){ //if there were decimals

    val = valSplit[0] + "." + valSplit[1]; //add decimals back

   }else{

    val = valSplit[0]; }



   return val;

  }

And in your jQuery, use like so:

$('.your-element').each(function(){

  $(this).html(commaSeparateNumber($(this).html()));

});

Here's the jsFiddle.