Allowing User Check / Uncheck All Checkboxes In Form

I am trying to allow a user to easily uncheck/check all checkboxes in a form by checking or unchecking a checkbox. The checkboxes would be in a form like so:
<form action="" method="post">
<input type="checkbox" name="checked[]" value="1">
<input type="checkbox" name="checked[]" value="2">
<input type="checkbox" name="checked[]" value="3">
<input type="checkbox" name="checked[]" value="4">
</form>

I use an event handler to call a function:
<input type="checkbox" name="checkall" onchange="checkall('checked[]', this.checked)">

And here is the function:
function checkall(box_name, current_state) {
all = document.getElementsByName(box_name);
action = (current_state == "checked") ? "false" : "true";
for (i = 0; i < all.length; i++) {
all[i].checked=action;
}}

The event handler should pass the name of the checkboxes and whether the "check all" checkbox is currently checked or not to the checkall() function. Nothing happens when I check/uncheck the "check all" function.