How to add event listeners in JS

<script>
function addListeners(){
    if(window.addEventListener) {
        document.getElementById('mybtn').addEventListener("click",btn1func,false);
        document.getElementById('mybtn2').addEventListener("mouseover",btn2func,false);
    } else if (window.attachEvent){ // Added For Inetenet Explorer versions previous to IE9
        document.getElementById('mybtn').attachEvent("onclick", btn1func);
        document.getElementById('mybtn2').attachEvent("onmouseover",btn2func);
    }
    // Write your functions here
    function btn1func(){
        alert(this.id+" : mouse-click makes script run");
    }
    function btn2func(){
        alert(this.id+" : mouse-over makes script run");
    }
}
window.onload = addListeners; 
</script>