<script>

    var strings = new Array("first","second","third");
    function press(event){
        if(event.keyCode== 13 ){
                var values = document.getElementById("text").value.split("\n");
                var str = values[values.length- 2 ];
                var in_array = false;
                for(i= 0 ;i<strings.length;i++){
                    if(strings[i]==str){
                        in_array = true;
                    }
                }
                if(!in_array) alert("Нету такого элемента!");
        }
    }
</script>

<textarea id="text" onkeyup="press(event)"></textarea>

 var values = document.getElementById("text").value.split("\n"); 

 var values = document.getElementById("text").value.split("\r\n"); 

                var str = values[values.length- 2 ];

		str = str.replace(/\r/,"");