Set CSS without atribute like .click

Set CSS without atribute like .click

Hello jquery-Forum,
My problem is that i have a div who is responsive in width.
The height have to be the same like the width.
How can i say in jquery that he have to set "("mydiv").css("height", width)"
without atribute like .click?
he have to do it automaticly without user-action.

Thank you and sorry for my bad English