firefox change height of textarea on enter not working

Question

I have been trying to change the height of textarea when the user hits enter so they do not have to scroll. i could mange it on IE and Chrome however i could not make it work on Firefox. Please have a look at my code. i am really new to this. it seems like it does not recognize event and i could not figure out a way around. Here is my code:

<form id="blog-comment-form" method="post" action="index.php">
<textarea id="comment" name="b_com" onkeyup="showmsg()"  placeholder="ADD YOUR COMMENT HERE"></textarea>
<input type="submit" name="submit" value="POST COMMENT"/>
</form>

i am calling the function from an external file. And my javascript code:

function showmsg() {

    if(!event){
           event= window.event;}
    if (event.keyCode==13) {
        var a = document.getElementById("comment");
    var b = document.getElementById("comment").style.scrollHeight;
    a.style.height = ((a.scrollHeight)+6) + "px"; 
}else {
    var a = document.getElementById("comment");
    var b = document.getElementById("testthis").style.height;
    a.style.height = ((a.scrollHeight)+6) + "px"; 
}

}

Thank You

Answer 1

Write this:

function showmsg(event) {

in place of:

function showmsg() {

AND

write this (in HTML markup):

onkeyup="showmsg(event)"

in place of:

onkeyup="showmsg()"