_menu.jade

nav
  ul
    li(class=selected == 'home' ? 'active':'') Home
    li(class=selected == 'blog' ? 'active':'') Blog
    li.dropdown
      a(href='#' class='dropdown-toggle' data-toggle='dropdown' role='button' aria-expanded='false')
        | Dropdown
        span.caret
      ul.dropdown-menu(role='menu')
        li: a(href='#') Action
        li: a(href='#') Another action
        li: a(href='#') Some other action
        li.divider
        li.dropdown-header Nav Header
        li: a(href='#') Separated link
        li: a(href='#') Another separate link

child.jade

extends parent

block link
  -var selected = 'blog';

parent.jade

doctype 5

html
  block link
    -var selected = 'home'; //default

  -var menu = { 'home': '/home', 'blog': '/blog', 'contact': '/contact' };

  body
    include _menu