<script>
window.addEventListener('load', function () {
document.getElementById("btn").click();
})
</script>
document.addEventListener("click", function(){
document.getElementById("demo").innerHTML = "Hello World";
});
document.addEventListener("mouseover", triggerBtnClick);
document.addEventListener("click", someOtherFunction);
document.addEventListener("mouseout", someOtherFunction);
<script>
function triggerBtnClick()
{
document.getElementById("btn").click();
}
</script