help me to resolve this checkbox problem

somewhere in the body tag I have this coding

<input type="hidden" name="hiddenName_#ContractList.ContractID#" value="#Description#" />
<input type="checkbox" name="contracts" value="#ContractList.ContractID#" />

the variable #ContractList.ContractID# comes from a query in coldfusion

and I have a header checkbox that trigger checkall() function

and the code of the function is

function checkAll()
{
var contractBoxes = document.ContractForm.contracts;

for (var i = 0; i < contractBoxes.length; ++i)
{
if (contractBoxes[i].checked == true)
{
contractBoxes[i].checked = false;
}
else
{
contractBoxes[i].checked = true;
}
}
}


the document.ContractForm.contracts; refer to a input form and contracts is a checkbox name

my problem is,

if there is only 1 row of record, the contractBoxes.length will become undefined and will not go into the for loop

if I force it by doing this

function checkAll()
{
var contractBoxes = document.ContractForm.contracts;

if (contractBoxes.length == null)
{
contractBoxes.length = 1;
}

for (var i = 0; i < contractBoxes.length; ++i)
{

if (contractBoxes[i].checked == true)
{
contractBoxes[i].checked = false;
}
else
{
contractBoxes[i].checked = true;
}
}
}

it goes into the for loop but it does not toggle my checkbox

just imagine this scenario with yahoo mailbox where there is a checkbox at the left side, you click the header checkbox, all the checkboxes will be checked,