Copy one TextBox value to another while typing in JavaScript

<form>
    <label for="textbox1">Textbox1:</label>
    <input type="text" id="textbox1" name="textbox1" onkeyup="copyText()" /><br />
    <br />
    <label for="textbox2">Textbox2:</label>
    <input type="text" id="textbox2" name="textbox2" /><br />
</form>
<script type="text/javascript">
    function copyText() {
        var textbox1 = document.getElementById("textbox1");
        var textbox2 = document.getElementById("textbox2");
        textbox2.value = textbox1.value;
    }
</script>