Javascript Currency Formatter

fmt_currency.js

function fmt_currency(num){
    var str=parseFloat(num).toFixed(2);
    var arr=str.split('');
    var out='';
    var char='';
    for(var x=0, len=arr.length;x<len;x++){
        if(x<=2){
            out=arr.pop()+out;
        }else{
            char=arr.pop();
            if((x-2)%3==1 && x!==3 && !isNaN(parseInt(char,10))){out=','+out;}
            out=char+out;
        }
    }
    return '$'+out;
}