"Check all" for checkboxes

Hi there!
I have a long list of checkboxes than I wanna add “check all”
If here havnt a solution “out of the box”
I already have a js script to do this, but for normal work of a cript, I need to add a class directly to the checkbox object, but I dont know how

http://ww.wp.xz.cn/plugins/contact-form-7/