Example Angular http service interceptor (from http://www.twilio.com/blog/2013/08/votr-part-4-angularjs-and-authentication-with-couchdb.html)

angular-http-interceptor.js

app.config(function($httpProvider) {
  $httpProvider.interceptors.push(function($rootScope, $location, $q) {
    return {
      'request': function(request) {
        // if we're not logged-in to the AngularJS app, redirect to login page
        $rootScope.loggedIn = $rootScope.loggedIn || $rootScope.username;
        if (!$rootScope.loggedIn && $location.path() != '/login') {
          $location.path('/login');       
        }
        return request;
      },
      'responseError': function(rejection) {
        // if we're not logged-in to the web service, redirect to login page
        if (rejection.status === 401 && $location.path() != '/login') {
          $rootScope.loggedIn = false;
          $location.path('/login');
        }
        return $q.reject(rejection);         
      }
    };
  });
});