playlist with if/else

<script type="text/javascript">  /*Diretório onde as musicas estão*/
                diretorio ="./musicas";
            musicas = ["http://n1.vmuzike.net/music/My_Chemical_Romance/The_Black_Parade/Cancer", "http://waaak.free.fr/root/sauvegarde/musiques/Simple%20Plan%20-%20Perfect", "http://join.haje.org/~adaline/favorites/music/Complicated","http://a.tumblr.com/Nxn6PbslMig5nggkUSYDbIEUo1","http://mp3.shmidt.net/files_play/2763285c44371a0f66442311ebb5fa88/mp3/T/Trapt/Trapt_-_Headstrong_mp3.shmidt.net.mp3","a","b","c","d","e","f"]; /*Aqui é a array que adiciona as músicas*/
            musicaAtual = 0;
                /*valor inicial da variável musicaAtual*/
            next = function() {
                 musicaAtual= musicaAtual + 1;
             if (musicaAtual > 4 ) {
                 document.getElementById("player-source").setAttribute("src", diretorio+musicas[musicaAtual]+".mp3");
                 document.getElementById("player").load();
            }
                 else  {
                     document.getElementById("player-source").setAttribute("src", musicas[musicaAtual]+".mp3");
                     document.getElementById("player").load(); }

                 back = function(){
                     musicaAtual= musicaAtual - 1;
                     document.getElementById("player-source").setAttribute("src", musicas[musicaAtual]+".mp3");
                     document.getElementById("player").load();
            }
            }

</script>