mithril-token-checker-with-plugin.js

// Uses the m.route.configure plugin in https://gist.github.com/impinball/2e1a78f24f1979f2b7eb
m.route.configure(function () {
  return hasLocalStorageToken()
}, function (route) {
  return route("/login")
})

mithril-token-checker.js

m.route = (function (old) {
  function check(route) {
    if ({}.toString.call(route) !== "[object String]" || hasLocalStorageToken()) {
      return old.apply(arguments)
    } else {
      return old("/login")
    }
  }
  // So the properties are still accessible (and doesn't break Mithril)
  for (var i in old) if ({}.hasOwnProperty.call(old, i)) {
    check[i] = old[i]
  }
  return check
})(m.route)