Question

I don’t get it. Why does this alert on page load? How to make it alert only “onfocus”?

<input id="mail" name="E-post" value="E-post">
<script>
window.onload = function () {
    function onfocusFunction() {
        alert('Why!?');
    }
    var myMail = document.getElementById('mail');
    myMail.onfocus = onfocusFunction();
}
</script>

Answer 1

With parenthesis (), you are calling onfocusFunction function and assigning undefined to myMail.onfocus:

myMail.onfocus = undefined;

as your onfocusFunction function does not return anything.

Without parenthesis, you will have onfocusFunction object, which should be assigned to .onfocus:

myMail.onfocus = onfocusFunction;  //No parenthesis before ';'

UPDATE: If you want to pass parameters either, then try like this:

function onfocusFunction(param1,param2) {
    return function(){
       alert(param1+":"+param2);
    };
}
var myMail = document.getElementById('mail');
myMail.onfocus = onfocusFunction("value1","value2");

Then when onfocus event will be triggered you will see "value1:value2" as an alert.