how to check checkbox status on change event

In my login page, I have a checkbox, that has to do something on change event,I have wrote change event in my js and I have to check whether it is checked or unchecked, I have used many methods like

$(".checkbox").is(":checked")/

$(".checkbox").attr("checked")/

$(".checkbox").prop("checked")

but all the above are not working, the last two return undefined where as the first one returns false always. I have fiddled this here .please help me to figure out the error