commfy0.js

function commfy0(n){
    var a = n.toString().split(".");
    var i = a[0].length%3;
    var prefix = (a[0].substr(0,i)+a[0].substr(i).replace(/(\d{3})/g,",$1")).replace(/^,/, "");
    //var postfix = a[1] ? "."+a[1].replace(/(\d{3})/g, "$1,").replace(/,$/, "") : "";
    return prefix+(a[1]?"."+a[1]:"");
}

commfy1.js

var commfy1 = (function(){
    var re=/(\d{1,3})(?=(?:\d{3})+(?:$|\.))/g;
    return function(n){
        var a=n.toString().split(".");
        a[0] = a[0].replace(re,"$1,");
        return a.join(".");
    };
})();

commfy2.js

var commfy2 = (function(){
    var re=/\B(?=(?:\d{3})+$)/g;
    return function(n){
        var a=n.toString().split(".");
        a[0] = a[0].replace(re,",");
        return a.join(".");
    };
})();

http://blog.hotoo.me/javascript-commfy.html