Javascript checkbox select all

<input name="dischk[]" type="checkbox">

<input name="disall" onclick="checkedAll(document.myform['dischk[]']);" type="checkbox" >

function checkedAll(field) {
if(document.myform.disall.checked)
{
for (i = 0; i < field.length; i++)
field[i].checked = true;
}
else
{
for (i = 0; i < field.length; i++)
field[i].checked = false;
}
}