[jQuery] need help: how to pass variable to attr in select statement

[jQuery] need help: how to pass variable to attr in select statement


code like below:
var selectid = Querystring.get("id");
$('div#mainnav .mainnav[alt="selectid?"]').addClass("color");
I want to select element which attr alt = selectid(variable from
querystring). How can I do that? Thanks.