Why am I getting undefined error in my array?

[Jen O]

<script>
        window.onload=myday;
 
            function myday(){
 
            var day=new Array("sunday","monday","tuesday","wednesday","thursday","friday","saturday");
 
            var d=new Date();
            theDay=d.getDay();
 
            for (i=0;i<=day.length;i++)
            {
                    if (i!=theDay)
                {
 
                document.write("today is not " + day[i]);
                document.write("<br />");}
 
                else
                {
                    document.write("today is " + day[i]);
                    document.write("<br />");}
            }
            }
 
 
</script>

[Dormilich]

because you use document.write().

if you call document.write() after the page finished loading (i.e. when onload is triggered) document.write() erases* the current document (completely) and writes the document anew. at that point all your JavaScript has been erased and thus your function is not defined.


* - it does that by implicitly calling document.open()

[Jen O]

How do you recommend I get around this? I have this example from my book(below), and it uses the onload with document.write. I believe this is why I am confused.

<script type="text/javascript">
    window.onload=initAll;
 
    function initAll(){
    var allmonths = new Array ("january","february","march","april","may","june","july","august","september","october","november","december")
    var birthmonth = prompt("what is your birth month?","");
    for(i=0;i<allmonths.length;i++){
        if (allmonths[i]==birthmonth){
            document.write("<strong>"+(i+1)+". "+allmonths[i]+" is your birth month!"+"</strong></br >");
            alert("Hooray "+allmonths[i]+" is your birth month!");
        } else{
            document.write((i+1)+". "+allmonths[i]+" is "+"<strong>"+"NOT"+"</strong>"+" your birth month!"+"</br >");
        }
    }
 
}
    </script>

[JKing]

It's a very small error. In your for loop you have for (i=0;i<=day.length;i++)

but if you compare this to your example for(i=0;i<allmonths.length;i++) you will notice the operator <= should be <.

You want your loop to terminate before 'i' is equal to the length. This is because arrays start at 0. If you have an array with length of 5 you would have indexes 0,1,2,3,4 but not 5.