JavaScript Alert if Statement

<script type="text/javascript">
<!--
var myNum = 3;

if(myNum == 3){
    document.write("Hello 3!");
}
//-->
</script>

JavaScript Alert else if Statement

<script type="text/javascript">
<!--
var myNum = 10;

if(myNum == 3){
    document.write("Hello 3!");
}else{  
    document.write("You're not 3...");  
}
//-->
</script>

Also remember that controlling the appearance of confirm() or alert() Javascript popups is not possible, because they are native controls.