[jQuery] how do you toggle a checkbox value?

[chris]

I know there is a quirk with checkboxes, but I am under the impression
that setting the check state to boolean values within javascript takes
care of that.  Unfortunately, this assumption is proving to be an
incorrect one.

// Here is a snippet of code
$("li#" + liId).click(function(event){

    var checkbox = $("#" + checkboxId)  // this is being found as
expected

    // flip the checkbox state
    console.log(checkbox.checked)
    checkbox.checked = !checkbox.checked

====== End

// This code snippet always returns Undefined even when the checkbox
is checked
console.log(checkbox.checked)

// which then makes the flip of the values not work
checkbox.checked = !checkbox.checked

What is the proper way to reference the checked value and flip the
value?

Thanks.