Changing TextArea Value

I'm trying to write a script that checks for when the enter button is pressed in a textarea and then appends a <br />. I have this:

function appendBreak (field, event) {
var keyCode = event.keyCode ? event.keyCode : event.which ? event.which : event.charCode;
if (keyCode == 13) {
document.formName.inputName.value = document.formName.inputName.value + '<br />'
alert('Captured Enter Key');
}
else
return true;
}

And in the particular element that i want to call this function I have this attribute. onkeypress="return appendBreak(this, event)
This code will work on regular text type inputs (i.e. <input type="text"....>). But I was having trouble identifying the value of a textarea.

This is what I currently have:function appendBreak (field, event) {
var keyCode = event.keyCode ? event.keyCode : event.which ? event.which : event.charCode;
if (keyCode == 13) {
//this.value = this.value + '<br />'
alert('whats up?');

alert(this.innerText);
this.innerText = this.innerText + '<br />'
alert(this.innerText);

}
else
return true;
}

It shows that what I am calling is undefined. But it does append the <br /> after the undefined. So if I could just locate the <textarea> value then I'd be okay.