element.onclick=function; element.attachEvent("onclick",function); element.addEventListener("click", fa, false);

<script type="text/javascript">

var link=document.getElementById("mylink");
link.onclick=function(){
  alert("I was clicked !");
};
/////////////////////////////////
    var fa = function () {
        alert("a");
    }
    //A
    var A = document.getElementById("a");
    A.attachEvent("onclick",fa);
/////////////////////////////////
    var fa = function () {
        alert("a");
    }
    //A
    var A = document.getElementById("a");
    A.addEventListener("click", fa, false);

</script>