Reverse a string in Javascript

gistfile1.js

String.prototype.reverse=function(){
    var that=this;//or this points to window
    function rev(len,result){
        if(len==0)
            return result;
        else{
            //console.log(this);
            return rev(len-1,result+that[len-1]);
        }
    }
    return rev(this.length,"");
}
'string'.reverse();//'gnirts'