生成一个session id

       //http://tools.ietf.org/html/rfc3548#section-4
       var genSessionId = function(length){
                var str = genSessionId.characters;
                if ( !"0"[0] ) { //fix IE67
                    str = str.split("");
                }
                for(var i=0,id="",len = str.length;i < length;i++){
                    id += str[Math.floor(Math.random() * len)];
                }
                return id;
            }
            genSessionId.characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_';
            alert(genSessionId(24))