Placing Value In Hidden Input Onsubmit

On submitting the form, I'm trying to place the items of a listbox in
a hidden input. It doesn't work. Is my approach correct?

I have a form like this:

<form name="myForm" onsubmit="return listBoxContents(this.Form.sel1)"
action="project_selection.php" method="post">

and a function as below:

function listBoxContents(sel1)
{
var selLength1 = sel1.options.length,i,values1=''

for(i=0; i<selLength1; i++)
values1+=sel1.options[i].value+ ",";

document.myForm.hiddenName.value = values1;

return true;
}

and the hidden value is as follows:

<input type=hidden name="hiddenName" value="initial">