Angular Style Guide

Table of Contents

1. High-level Goals
2. Third-party Dependencies
3. Directory and File Structure
4. Parts of Angular
5. Testing
6. General Patterns and Anti-patterns

High-level Goals

The principles we use to guide low-level decision making are:

1. Prioritise readability.
2. Be explicit, not implicit.
3. Favour composability over inheritance.
4. Think forward – ES6 and Web Components (Angular 2.0).
5. Know when to deviate from the style guide.

Example ES6 Application Scaffold

ES6 + AngularJS Application

Third-party Dependencies

1. SystemJS

• Why: SystemJS is an ES6 module loader that enables us to load assets in development and transpile ES6 to ES5 in production.

 import {dialogControllerModule} from './dialog.controller';
 import template from './dialog.template.html!text';

2. Traceur

• Why: Traceur is a transpiler supported by SystemJS. It lets us use ECMAScript 6 features before they are implemented in browsers.

3. Lo-Dash

• Why: lodash is a utility library we use throughout our application. Our use of _.extend could be replaced by Angular’s built in method angular.extend.

4. AngularUI Router

• Why: ui-router replaces Angular’s ngRoute module, and is built around states instead of URL routes, enabling nested views. Our use of $stateProvider could be replaced by $routeProvider.

Note: For a complete setup see our Example ES6 Application Scaffold.

Directory and File Structure

We organise our code as follows:

Folder structure

/app
  /components
    /alert
      alert.directive.js
      alert.directive.spec.js
      alert.template.html
  /config
    main.config.js
  /constants
    api-url.constant.js
  /routes
    /customers
      /index
        customers-index.template.html
        customers-index.route.js
        customers-index.controller.js
        customers-index.e2e.js
  /helpers
    /currency
      currency-filter.js
      currency-filter.spec.js
    /unit
    /e2e
  /services
    /creditors
      creditors.js
      creditors.spec.js
  bootstrap.js
  main.js
/assets
  /fonts
  /images
  /stylesheets
404.html
index.html

Specs (Unit/E2E)

Keep spec files in the same folder as the code being tested.

Components

Components are encapsulated DOM components. Each component contains all the HTML, CSS, JavaScript, and other dependencies needed to render itself.

Routes

A view, made up of components and unique pieces of UI, that points to a URL. Like components, each route contains all the HTML, CSS, JavaScript, and other dependencies needed to render itself.

Services

Services contain Business logic. For example, $http abstractions.

Config

Configures Providers. For example, $locationProvider.html5Mode(true);.

Constants

Although JavaScript does not yet support constants, we run our application through Traceur, which supports const.

The constant should be named in all uppercase if it's a global constant that will be used across many different functions. For example, export const API_URL = 'https://api.gocardless.com'.

If the constant is defined within a single function, it should be in regular camelCase.

Helpers

Pure functions. For example, a currencyFilter might take in a number and format it for a certain currency. Helpers take input and return output without having any side effects.

Parts of Angular

Rules for using each of the core parts of AngularJS (routes, directives, controllers, modules, and templates).

Routes

Use resolvers to inject data.

Why: The page is rendered only when all data is available. This means views are only rendered once all the required data is available, and you avoid the user seeing any empty views whilst the data is loading.

// Recommended
$stateProvider.state('customers.show', {
  url: '/customers/:id',
  template: template,
  controller: 'CustomersShowController',
  controllerAs: 'ctrl',
  resolve: {
    customer: [
      'Customers',
      '$stateParams',
      function customerResolver(Customers, $stateParams) {
        return Customers.findOne({
          params: { id: $stateParams.id }
        });
      }
    ]
  }
});

// Avoid
// Note: Controller written inline for the example
$stateProvider.state('customers.show', {
  url: '/customers/:id',
  template: template,
  controllerAs: 'ctrl',
  controller: [
    'Customers',
    '$stateParams',
    function CustomersShowController(Customers, $stateParams) {
      const ctrl = this;
      Customers.findOne({
        params: { id: $stateParams.id }
      }).then(function(customers) {
        ctrl.customers = customers;
      });
    }
  ]
});

Use query parameters to store route state. For example, the current offset and limit when paginating.

Why: The current view should be accurately reflected in the URL, which means any page refresh puts the user back in the exact state they were in.

// Recommended
function nextPage() {
  const currentOffset = parseInt($stateParams.offset, 10) || 0;
  const limit = parseInt($stateParams.limit, 10) || 10;
  const nextOffset = currentOffset + limit;
  Payments.findAll({
    params: { customers: $stateParams.id, limit: limit, offset: nextOffset }
  });
}

// Avoid
// Keeping route state in memory only
let currentOffset = 0;
const limit = 10;

function nextPage() {
  const nextOffset = currentOffset + limit;
  currentOffset = nextOffset;
  Payments.findAll({
    params: { customers: $stateParams.id, limit: limit, offset: nextOffset }
  });
}

Directives

Directive names must only contain a-z and at least one dash (-).

Why: Custom elements must have a dash (namespace) to differentiate them from native elements and prevent future component collisions.

<!-- Recommended -->
<dialog-box></dialog-box>
<button click-toggle="isActive"></button>

<!-- Avoid -->
<dialog></dialog>
<button toggle="isActive"></button>

Use element directives when content is injected, else use attribute directives.

Why: Separates responsibility: element directives add content; attribute directives add behaviour; class attributes add style.

<!-- Recommended -->
<alert-box message="Error"></alert-box>
<!-- Replaced with: -->
<alert-box message="Error" class="ng-isolate-scope">
  <div class="alert-box">
    <span class="alert-box__message">Error</span>
  </div>
</alert-box>

<!-- Avoid -->
<p alert-box message="Error"></p>
<!-- Replaced with: -->
<p alert-box message="Error">
  <div class="alert-box">
    <span class="alert-box__message">Error</span>
  </div>
</p>

<!-- Recommended -->
<button prevent-default="click">Submit</button>

<!-- Avoid -->
<prevent-default event="click">Submit</prevent-default>

Use an isolate scope for element directives. Share scope for attribute directives.

Why: Using an isolate scope forces you to expose an API by giving the component all the data it needs. This increases reusability and testability. When using a shared scope for attribute directives you should not write to it or rely on any existing data. Attribute directives should not have an isolate scope because doing so overwrites the current scope.

// Recommended
angular.module('alertListComponentModule', [])
  .directive('alertList', [
    function alertListDirective() {
      return {
        restrict: 'E',
        scope: {}
      };
    }
  ]);

// Avoid
angular.module('alertListComponentModule', [])
  .directive('alertList', [
    function alertListDirective() {
      return {
        restrict: 'E'
      };
    }
  ]);

// Recommended
angular.module('alertListComponentModule', [])
  .directive('alertList', [
    function alertListDirective() {
      return {
        restrict: 'A'
      };
    }
  ]);

// Avoid
angular.module('alertListComponentModule', [])
  .directive('alertList', [
    function alertListDirective() {
      return {
        restrict: 'A',
        scope: {}
      };
    }
  ]);

angular.module('alertListComponentModule', [])
.directive('alertList', [
  function alertListDirective() {
    return {
      restrict: 'A',
      scope: true
    };
  }
]);

When using isolate-scope properties, always bindToController.

Why: It explicitly shows what variables are shared via the controller.

// Recommended
angular.module('alertListComponentModule', [])
  .directive('alertList', [
    function alertListDirective() {
      return {
        restrict: 'E',
        controller: 'AlertListController',
        controllerAs: 'ctrl',
        bindToController: true,
        template: template,
        scope: {}
      };
    }
  ]);

// Avoid
angular.module('alertListComponentModule', [])
  .directive('alertList', [
    function alertListDirective() {
      return {
        restrict: 'E',
        controller: 'AlertListController',
        template: template,
        scope: {}
      };
    }
  ]);

Tear down directives, subscribe to $scope.$on('$destroy', ...) to get rid of any event listeners or DOM nodes created outside the directive element.

Why: It avoids memory leaks and duplicate event listeners being bound when the directive is re-created.

// Recommended
angular.module('adminExpandComponentModule', [])
  .directive('adminExpand', [
    '$window',
    function adminExpand($window) {
      return {
        restrict: 'A',
        scope: {},
        link: function adminExpandLink(scope, element) {
          function expand() {
            element.addClass('is-expanded');
          }

          $window.document.addEventListener('click', expand);

          scope.$on('$destroy', function onAdminExpandDestroy() {
            $window.document.removeEventListener('click', expand);
          });
        }
      };
    }
  ]);

// Avoid
angular.module('adminExpandComponentModule', [])
  .directive('adminExpand', [
    '$window',
    function adminExpand($window) {
      return {
        restrict: 'A',
        scope: {},
        link: function adminExpandLink(scope, element) {
          function expand() {
            element.addClass('is-expanded');
          }

          $window.document.addEventListener('click', expand);
        }
      };
    }
  ]);

Anti-Patterns

• Don't rely on jQuery selectors. Use directives to target elements instead.
• Don't use jQuery to generate templates or DOM. Use directive templates instead.
• Don't prefix directive names with x-, polymer-, ng-.

Controllers

Use controllerAs syntax.

Why: It explicitly shows what controller a variable belongs to, by writing {{ ctrl.foo }} instead of {{ foo }}.

// Recommended
$stateProvider.state('authRequired.customers.show', {
  url: '/customers/:id',
  template: template,
  controller: 'CustomersShowController',
  controllerAs: 'ctrl'
});

// Avoid
$stateProvider.state('authRequired.customers.show', {
  url: '/customers/:id',
  template: template,
  controller: 'CustomersShowController'
});

// Recommended
angular.module('alertListComponentModule', [])
  .directive('alertList', [
    function alertListDirective() {
      return {
        restrict: 'E',
        controller: 'AlertListController',
        controllerAs: 'ctrl',
        bindToController: true,
        template: template,
        scope: {}
      };
    }
  ]);

// Avoid
angular.module('alertListComponentModule', [])
  .directive('alertList', [
    function alertListDirective() {
      return {
        restrict: 'E',
        controller: 'AlertListController',
        template: template,
        scope: {}
      };
    }
  ]);

Inject ready data instead of loading it in the controller.

Why:

• 2.1. Simplifies testing with mock data.
• 2.2. Separates concerns: data is resolved in the route and used in the controller.

// Recommended
angular.module('customersShowControllerModule', [])
  .controller('CustomersShowController', [
    'customer', 'payments', 'mandates',
    function CustomersShowController(customer, payments, mandates){
      const ctrl = this;

      _.extend(ctrl, {
        customer: customer,
        payments: payments,
        mandates: mandates
      });
    }
  ]);

// Avoid
angular.module('customersShowControllerModule', [])
  .controller('CustomersShowController', [
    'Customers', 'Payments', 'Mandates', '$stateParams',
    function CustomersShowController(Customers, Payments, Mandates, $stateParams){
      const ctrl = this;

      Customers.findOne({
        params: { id: $stateParams.id }
      }).then(function(customers) {
        ctrl.customers = customers;
      });

      Payments.findAll().then(function(payments) {
        ctrl.payments = payments;
      });

      Mandates.findAll().then(function(mandates) {
        ctrl.mandates = mandates;
      });
    }
  ]);

Extend a controller’s properties onto the controller.

Why: What is being exported is clear and always done in one place, at the bottom of the file.

// Recommended
angular.module('organisationRolesNewControllerModule', [])
  .controller('OrganisationRolesNewController', [
    'permissions',
    function CustomersShowController(permissions){
      const ctrl = this;

      function setAllPermissions(access) {
        ctrl.form.permissions.forEach(function(permission) {
          permission.access = access;
        });
      }

      _.extend(ctrl, {
        permissions: permissions,
        setAllPermissions: setAllPermissions
      });
    }
  ]);

// Avoid
angular.module('organisationRolesNewControllerModule', [])
  .controller('OrganisationRolesNewController', [
    'permissions',
    function CustomersShowController(permissions){
      const ctrl = this;

      ctrl.permissions = permissions;

      ctrl.setAllPermissions = function setAllPermissions(access) {
        ctrl.form.permissions.forEach(function(permission) {
          permission.access = access;
        });
      }
    }
  ]);

Only extend the controller with properties used in templates.

Why: Adding unused properties to the digest cycle is expensive.

// Recommended
angular.module('webhooksIndexControllerModule', [])
  .controller('WebhooksIndexController', [
    'TestWebhooks', 'AlertList', 'webhooks'
    function WebhooksIndexController(TestWebhooks, AlertList, webhooks) {
      const ctrl = this;

      function success() {
        AlertList.success('Your test webhook has been created and will be sent shortly');
      }

      function error() {
        AlertList.error('Failed to send test webhook, please try again');
      }

      function sendTestWebhook(webhook) {
        TestWebhooks.create({
          data: { test_webhooks: webhook }
        }).then(success, error);
      }

      _.extend(ctrl, {
        webhooks: webhooks,
        sendTestWebhook: sendTestWebhook
      });
    }
  ]);

// Avoid
angular.module('webhooksIndexControllerModule', [])
  .controller('WebhooksIndexController', [
    'TestWebhooks', 'AlertList', 'webhooks'
    function WebhooksIndexController(TestWebhooks, AlertList, webhooks) {
      const ctrl = this;

      function success() {
        AlertList.success('Your test webhook has been created and will be sent shortly');
      }

      function error() {
        AlertList.error('Failed to send test webhook, please try again');
      }

      function sendTestWebhook(webhook) {
        TestWebhooks.create({
          data: { test_webhooks: webhook }
        }).then(success, error);
      }

      _.extend(ctrl, {
        webhooks: webhooks,
        success: success,
        error: error,
        sendTestWebhook: sendTestWebhook
      });
    }
  ]);

Store presentation logic in controllers and business logic in services.

Why:

• 5.1. Simplifies testing business logic.
• 5.2. Controllers are glue code, and therefore require integration tests, not unit tests.

// Recommended
angular.module('webhooksControllerModule', [])
.controller('WebhooksController', [
  'TestWebhooks',
  function WebhooksController(TestWebhooks) {
    const ctrl = this;

    function sendTestWebhook(webhook) {
      TestWebhooks.create({
        data: { test_webhooks: webhook }
      }).then(function() {
        $state.go('authRequired.organisation.roles.index', null);
        AlertList.success('Your test webhook has been created and will be sent shortly');
      });
    }

    _.extend(ctrl, {
      sendTestWebhook: sendTestWebhook
    });
  }
]);

// Avoid
angular.module('webhooksControllerModule', [])
.controller('WebhooksController', [
  '$http',
  function WebhooksController($http) {
    const ctrl = this;

    function sendTestWebhook(webhook) {
      $http({
        method: 'POST',
        data: { test_webhooks: webhook },
        url: '/test_webhooks'
      });
    }

    _.extend(ctrl, {
      sendTestWebhook: sendTestWebhook
    });
  }
]);

Only instantiate controllers through routes or directives.

Why: Allows reuse of controllers and encourages component encapsulation.

// Recommended
angular.module('alertListComponentModule', [])
  .directive('alertList', [
    function alertListDirective() {
      return {
        restrict: 'E',
        controller: 'AlertListController',
        controllerAs: 'ctrl',
        bindToController: true,
        template: template,
        scope: {}
      };
    }
  ]);

<!-- Avoid -->
<div ng-controller='AlertListController as ctrl'>
  <span>{{ ctrl.message }}</span>
</div>

Anti-Patterns

• Don’t manipulate DOM in your controllers, this will make them harder to test. Use directives instead.

Services and Factories

Treat Service objects like a Class with static methods; don't export services as a single function

Why: easier to see at the definition site and the call site exactly what function the service provides.

// Recommend
angular.module('buildCSVModule', []).factory('BuildCSV', function buildCVS() {
  function build() {
    // ...
  }

  return {
    build: build,
  }
});

// Avoid
angular.module('buildCSVModule', []).factory('BuildCSV', function buildCSV() {
  function build() {
    // ...
  }

  return build;
});

Services that take in data and manipulate it should never mutate the original object and return the new object

Why: Avoiding mutation in service objects makes it possible to reason about them from their call site without knowing what they do internally.

// Recommend
let events = [...];
events = EventPresenterService.present(events);

// Avoid
const events = [...];
EventPresenterService.present(events);
// events has been mutated

Prefer ImmutableJS when creating services that manipulate data

Use fromJS to convert a JS object into an Immutable type, and toJS at the end to convert back.

Why: ImmutableJS is a fantastic library for taking data and manipulating it without ever mutating.

// Recommend
angular.module('eventPresenterModule', []).factory('EventPresenterService', function eventPresenterService() {
  function present(events) {
    return Immutable.fromJS(events).map(function(event) {
      event.set('some_data', true);
    }).map(function(event) {
      ...
    }).toJS();
  }

  return {
    present: present,
  }
});

Modules

Name a module using lowerCamelCase and append Module.

Why: A module name should be mapped to a file and clearly differentiated from constructors and service objects.

// Recommended
angular.module('usersPasswordEditControllerModule', [])
  .controller('UsersPasswordEditController', []);

// Avoid
angular.module('UsersPasswordEditControllerModule', [])
  .controller('UsersPasswordEditController', []);

Create one module per file and don’t alter a module other than where it is defined.

Why:

• 1.1. Prevents polluting the global scope.
• 1.2. Simplifies unit testing by declaring all dependencies needed to run each module.
• 1.3. Negates necessity to load files in a specific order.

// Recommended
angular.module('usersPasswordEditControllerModule', [])
  .controller('UsersPasswordEditController', []);

// Avoid
angular.module('app')
  .controller('UsersPasswordEditController', []);

Use ES6 module system and reference other modules using Angular Module’s name property.

Why:

• 2.1. Encapsulates all required files, making unit testing easier and error feedback more specific.
• 2.2. Simplifies upgrading to Angular 2.0, which uses ES6 modules.

// Recommended
import {passwordResetTokensModule} from 'app/services/password-reset-tokens/password-reset-tokens';
import {sessionModule} from 'app/services/session/session';
import {alertListModule} from 'app/components/alert-list/alert-list';

export const usersPasswordEditControllerModule = angular.module('usersPasswordEditControllerModule', [
  passwordResetTokensModule.name,
  sessionModule.name,
  alertListModule.name
]);

// Avoid
import {passwordResetTokensModule} from 'app/services/password-reset-tokens/password-reset-tokens';
import {sessionModule} from 'app/services/session/session';
import {alertListModule} from 'app/components/alert-list/alert-list';

export const usersPasswordEditControllerModule = angular.module('usersPasswordEditControllerModule', [
  'passwordResetTokensModule',
  'sessionModule',
  'alertListModule'
]);

Use relative imports only when importing from the current directory or any of its children. Use absolute paths when referencing modules in parent directories.

Why: Makes it easier to edit directories.

// Current directory: app/services/creditors/

// Recommended
import {API_URL} from 'app/constants/api-url.constant';
import {authInterceptorModule} from 'app/services/auth-interceptor/auth-interceptor';
import {organisationIdInterceptorModule} from 'app/services/organisation-id-interceptor/organisation-id-interceptor';

// Avoid
import {API_URL} from '../../constants/api-url.constant';
import {authInterceptorModule} from '../services/auth-interceptor/auth-interceptor';
import {organisationIdInterceptorModule} from '../services/organisation-id-interceptor/organisation-id-interceptor';

Templates

Use the one-time binding syntax when data does not change after first render.

Why: Avoids unnecessary expensive $watchers.

<!-- Recommended -->
<p>Name: {{::ctrl.name}}</p>

<!-- Avoid -->
<p>Name: {{ctrl.name}}</p>

Anti-Patterns

• Don’t use ngInit – use controllers instead.
• Don’t use <div ng-controller="Controller"> syntax. Use directives instead.

Testing

Our applications are covered by two different types of test:

• unit tests, which test individual components by asserting that they behave as expected.
• End to End, or E2E, tests, which load up the application in a browser and interact with it as if a user would, asserting the application behaves expectedly.

To write our tests we use Jasmine BDD and ngMock.

Unit Testing

Every component should have a comprehensive set of unit tests.

Structure of Unit Tests

Tests should be grouped into logical blocks using Jasmine's describe function. Tests for a function should all be contained within a describe block, and describe blocks should also be used to describe different scenarios, or contexts:

describe('#update', function() {
  describe('when the data is valid', function() {
    it('shows the success message', function() {…});
  });

  describe('when the data is invalid', function() {
    it('shows errors', function() {…});
  });
});

Dependencies

Each component should have its dependencies stubbed in each test.

Inject the dependencies and the components being tested in a beforeEach function. This encapsulates each test's state, ensuring that they are independent, making them easier to reason about. Tests should never depend on being run in a specific order.

let SomeService;

beforeEach(inject(function($injector) {
  SomeService = $injector.get('SomeService');
}));

Controllers

When injecting controllers for a test, use the controller as syntax:

beforeEach(inject(function($injector, $controller) {
  $controller('OrganisationController as ctrl', {…});
}));

Always create a new scope to pass into the controller:

let scope;
let organisation = {
  name: 'GoCardless'
};

beforeEach(inject(function($injector, $controller) {
  scope = $injector.get('$rootScope').$new();
  $controller('OrganisationController as ctrl', {
    $scope: scope,
    organisation: organisation
  });
}));

Fixtures

When stubbing an API request using $httpBackend, always respond with a correctly formatted object. These responses should be saved individually as .json files and imported using the SystemJS JSON plugin:

import updateFixture from 'app/services/roles/update.fixture.json!json';

$httpBackend.expectPUT('someurl.com').respond(201, updateFixture);

General Patterns and Anti-Patterns

Rules that pertain to our application at large, not a specific part of Angular.

Patterns

Angular abstractions

Use:

• $timeout instead of setTimeout
• $interval instead of setInterval
• $window instead of window
• $document instead of document
• $http instead of $.ajax
• $q (promises) instead of callbacks

Why: This makes tests easier to follow and faster to run as they can be executed synchronously.

Dependency injection annotations

Always use array annotation for dependency injection and bootstrap with strictDi.

Why: Negates the need for additional tooling to guard against minification and strictDi throws an error if the array (or $inject) syntax is not used.

// Recommended
angular.module('creditorsShowControllerModule', [])
  .controller('CreditorsShowController', [
    'creditor', 'payments', 'payouts',
    function CreditorsShowController(creditor, payments, payouts) {
      const ctrl = this;

      _.extend(ctrl, {
        creditor: creditor,
        payments: payments,
        payouts: payouts
      });
    }
  ]);

// Avoid
angular.module('creditorsShowControllerModule', [])
  .controller('CreditorsShowController',
    function CreditorsShowController(creditor, payments, payouts) {
      const ctrl = this;

      _.extend(ctrl, {
        creditor: creditor,
        payments: payments,
        payouts: payouts
      });
    });

// Recommended
import {mainModule} from './main';

angular.element(document).ready(function() {
  angular.bootstrap(document.querySelector('[data-main-app]'), [
    mainModule.name
  ], {
    strictDi: true
  });
});

// Avoid
import {mainModule} from './main';

angular.element(document).ready(function() {
  angular.bootstrap(document.querySelector('[data-main-app]'), [
    mainModule.name
  ]);
});

Anti-patterns

Don’t use the $ name space in property names (e.g. $scope.$isActive = true).

Why: Makes clear what is an Angular internal.

Don't use globals. Resolve all dependencies using Dependency Injection.

Why: Using DI makes testing and refactoring easier.

Don't do if (!$scope.$$phase) $scope.$apply(), it means your $scope.$apply() isn't high enough in the call stack.

Why: You should $scope.$apply() as close to the asynchronous event binding as possible.

Credits

We referred to lots of resources during the creation of this styleguide, including:

• Todd Motto’s styleguide
• John Papa’s styleguide
• Minko Gechev’s styleguide
• Google AngularJS and Closure styleguide
• The Angular.js GitHub wiki
• Digging into Angular’s “Controller as” syntax, by Todd Motto
• Python PEP 8 Styleguide