How to redirect in a ui-router resolver?

You can return a promise from your resolver function that will indicate whether to continue navigating to the state or not. If you decide to navigate somewhere else - reject the promise and specify the proper state:

resolver($q, $state, $timeout, auth) {
    var deferred = $q.defer();

    // $timeout is an example; it also can be an xhr request or any other async function
    $timeout(function() {
      if (!auth.isLoggedIn()) {
        // user is not logged, do not proceed
        // instead, go to a different page
        $state.go('noLoggedInPath');
        deferred.reject();
      } else {
        // everything is fine, proceed
        deferred.resolve();
      }
    });

    return deferred.promise;
}

Plunkr here.

UPD: updated code and added plunkr. Looks like it only works if you put it inside a timeout function.

Yauheni's answer does work, but to get around the weird timeout thing, you can reject the promise, and catch that in the $stateChangeError event, and do your redirect there. Like so...

state('admin', {
  resolve: {
    auth: function(UserService, $q, permissionService) {
      var deferred = $q.defer();
      return UserService.load().then(function(user){
        if (permissionService.can(user, {goTo: state})) {
          return deferred.resolve({});
        } else {
          return deferred.reject({redirectTo: 'some_other_state'});
        }
      });
    }
  }
});

And then ui-router always broadcasts "stateChangeError", so you can do something like this..

$rootScope.$on('$stateChangeError', function(evt, to, toParams, from, fromParams, error) {
  if (error.redirectTo) {
    $state.go(error.redirectTo);
  } else {
    $state.go('error', {status: error.status})
  }
})