Question

I am trying to add a class when a checkbox is checked.

My jquery:

$('input').attr("checked").change(function(){
$('div.menuitem').addClass("menuitemshow");
})

Answer 1

You should not use $("input") to select a checkbox, input will select all inputs. Instead you can use input:checkbox:

$('input:checkbox').change(function(){
    if($(this).is(":checked")) {
        $('div.menuitem').addClass("menuitemshow");
    } else {
        $('div.menuitem').removeClass("menuitemshow");
    }
});

Basically what this does is execute whatever is inside the function(){} when the checkbox is changed. Then you can just use jQuery is to check if the checkbox is checked or not..