How To Format Numbers In Calculator Application

I am creating a calculator app and I'm a bit stuck with number formatting. Example of how I would like a number to be formatted: $2,899,799
I currently have the formatting working except for the commas. I'm using the following code to achieve this. My result is currently showing up like this $2899799

function format (expr, decplaces) {
var str = "" + Math.round (eval(expr) * Math.pow(10,decplaces))
while (str.length <= decplaces) {
str = "0" + str
} var decpoint = str.length - decplaces
return str.substring(0,decpoint) + str.substring(decpoint,str.length);
}

function dollarize (expr) {
return "$" + format(expr,0)
//RESULT FIELDS
form.AnnualScrapSales.value = dollarize(((A - B) * (C2/100)) * G2)
Is there a way to add to this "function format" code to also achieve the commas?