when i run this code on asp.net content page then it's not working and error message comes that "0x800a1391 - JavaScript runtime error: 'btn' is undefined" but when i run this code on html page then it's execute without any errors.
 
<input type="button" value="Click" id="btn" />
        <script type="text/javascript">
            btn.addEventListener("mouseover", changeColorOnMouseOver, false);
            btn.addEventListener("mouseout", changeColorOnMouseOut, false);
            function changeColorOnMouseOver() {
                this.style.background = 'red';
                this.style.color = 'yellow';
            }
            function changeColorOnMouseOut() {
                this.style.background = 'black';
                this.style.color = 'white';
            }
        </script>