@ngdoc overview
@name Decorators
@sortOrder 345
@description

# Decorators in AngularJS

<div class="alert alert-warning">
  **NOTE:** This guide is targeted towards developers who are already familiar with AngularJS basics.
  If you're just getting started, we recommend the {@link tutorial/ tutorial} first.
</div>

## What are decorators?

Decorators are a design pattern that is used to separate modification or *decoration* of a class without modifying the
original source code. In AngularJS, decorators are functions that allow a service, directive or filter to be modified
prior to its usage.

## How to use decorators

There are two ways to register decorators

- `$provide.decorator`, and
- `module.decorator`

Each provide access to a `$delegate`, which is the instantiated service/directive/filter, prior to being passed to the
service that required it.

### $provide.decorator

The {@link api/auto/service/$provide#decorator decorator function} allows access to a $delegate of the service once it
has been instantiated. For example:

```js
angular.module('myApp', [])

.config([ '$provide', function($provide) {

  $provide.decorator('$log', [
    '$delegate',
    function $logDecorator($delegate) {

      var originalWarn = $delegate.warn;
      $delegate.warn = function decoratedWarn(msg) {
        msg = 'Decorated Warn: ' + msg;
        originalWarn.apply($delegate, arguments);
      };

      return $delegate;
    }
  ]);
}]);
```

After the `$log` service has been instantiated the decorator is fired. The decorator function has a `$delegate` object
injected to provide access to the service that matches the selector in the decorator. This `$delegate` will be the
service you are decorating. The return value of the function *provided to the decorator* will take place of the service,
directive, or filter being decorated.

<hr>

The `$delegate` may be either modified or completely replaced. Given a service `myService` with a method `someFn`, the
following could all be viable solutions:


#### Completely Replace the $delegate
```js
angular.module('myApp', [])

.config([ '$provide', function($provide) {

  $provide.decorator('myService', [
    '$delegate',
    function myServiceDecorator($delegate) {

      var myDecoratedService = {
        // new service object to replace myService
      };
      return myDecoratedService;
    }
  ]);
}]);
```

#### Patch the $delegate
```js
angular.module('myApp', [])

.config([ '$provide', function($provide) {

  $provide.decorator('myService', [
    '$delegate',
    function myServiceDecorator($delegate) {

      var someFn = $delegate.someFn;

      function aNewFn() {
        // new service function
        someFn.apply($delegate, arguments);
      }

      $delegate.someFn = aNewFn;
      return $delegate;
    }
  ]);
}]);
```

#### Augment the $delegate
```js
angular.module('myApp', [])

.config([ '$provide', function($provide) {

  $provide.decorator('myService', [
    '$delegate',
    function myServiceDecorator($delegate) {

      function helperFn() {
        // an additional fn to add to the service
      }

      $delegate.aHelpfulAddition = helperFn;
      return $delegate;
    }
  ]);
}]);
```

<div class="alert alert-info">
  Note that whatever is returned by the decorator function will replace that which is being decorated. For example, a
  missing return statement will wipe out the entire object being decorated.
</div>

<hr>

Decorators have different rules for different services. This is because services are registered in different ways.
Services are selected by name, however filters and directives are selected by appending `"Filter"` or `"Directive"` to
the end of the name. The `$delegate` provided is dictated by the type of service.

| Service Type | Selector                      | $delegate                                                             |
|--------------|-------------------------------|-----------------------------------------------------------------------|
| Service      | `serviceName`                 | The `object` or `function` returned by the service                    |
| Directive    | `directiveName + 'Directive'` | An `Array.<DirectiveObject>`<sub>{@link guide/decorators#drtvArray 1}</sub> |
| Filter       | `filterName + 'Filter'`       | The `function` returned by the filter                                 |

<small id="drtvArray">1. Multiple directives may be registered to the same selector/name</small>

<div class="alert alert-warning">
  **NOTE:** Developers should take care in how and why they are modifying the `$delegate` for the service. Not only
  should expectations for the consumer be kept, but some functionality (such as directive registration) does not take
  place after decoration, but during creation/registration of the original service. This means, for example, that
  an action such as pushing a directive object to a directive `$delegate` will likely result in unexpected behavior.

  Furthermore, great care should be taken when decorating core services, directives, or filters as this may unexpectedly
  or adversely affect the functionality of the framework.
</div>

### module.decorator

This {@link api/ng/type/angular.Module#decorator function} is the same as the `$provide.decorator` function except it is
exposed through the module API. This allows you to separate your decorator patterns from your module config blocks.

Like with `$provide.decorator`, the `module.decorator` function runs during the config phase of the app. That means
you can define a `module.decorator` before the decorated service is defined.

Since you can apply multiple decorators, it is noteworthy that decorator application always follows order
of declaration:

- If a service is decorated by both `$provide.decorator` and `module.decorator`, the decorators are applied in order:

```js
angular
  .module('theApp', [])
  .factory('theFactory', theFactoryFn)
  .config(function($provide) {
    $provide.decorator('theFactory', provideDecoratorFn); // runs first
  })
  .decorator('theFactory', moduleDecoratorFn); // runs seconds
```

- If the service has been declared multiple times, a decorator will decorate the service that has been declared
last:

```js
angular
  .module('theApp', [])
  .factory('theFactory', theFactoryFn)
  .decorator('theFactory', moduleDecoratorFn)
  .factory('theFactory', theOtherFactoryFn);

// `theOtherFactoryFn` is selected as 'theFactory' provider and it is decorated via `moduleDecoratorFn`.
```

## Example Applications

The following sections provide examples each of a service decorator, a directive decorator, and a filter decorator.

### Service Decorator Example

This example shows how we can replace the $log service with our own to display log messages.

<example module="myServiceDecorator" name="service-decorator">
  <file name="script.js">
    angular.module('myServiceDecorator', []).

      controller('Ctrl', [
        '$scope',
        '$log',
        '$timeout',
        function($scope, $log, $timeout) {
          var types = ['error', 'warn', 'log', 'info' ,'debug'], i;

          for (i = 0; i < types.length; i++) {
            $log[types[i]](types[i] + ': message ' + (i + 1));
          }

          $timeout(function() {
            $log.info('info: message logged in timeout');
          });
        }
      ]).

      directive('myLog', [
        '$log',
        function($log) {
          return {
            restrict: 'E',
            template: '<ul id="myLog"><li ng-repeat="l in myLog" class="{{l.type}}">{{l.message}}</li></ul>',
            scope: {},
            compile: function() {
              return function(scope) {
                scope.myLog = $log.stack;
              };
            }
          };
        }
      ]).

      config([
        '$provide',
        function($provide) {

          $provide.decorator('$log', [
            '$delegate',
            function logDecorator($delegate) {

              var myLog = {
                warn: function(msg) {
                  log(msg, 'warn');
                },
                error: function(msg) {
                  log(msg, 'error');
                },
                info: function(msg) {
                  log(msg, 'info');
                },
                debug: function(msg) {
                  log(msg, 'debug');
                },
                log: function(msg) {
                  log(msg, 'log');
                },
                stack: []
              };

              function log(msg, type) {
                myLog.stack.push({ type: type, message: msg.toString() });
                if (console && console[type]) console[type](msg);
              }

              return myLog;

            }
          ]);

        }
      ]);
  </file>

  <file name="index.html">
    <div ng-controller="Ctrl">
      <h1>Logs</h1>
      <my-log></my-log>
    </div>
  </file>

  <file name="style.css">
    li.warn { color: yellow; }
    li.error { color: red; }
    li.info { color: blue }
    li.log { color: black }
    li.debug { color: green }
  </file>

  <file name="protractor.js" type="protractor">
    it('should display log messages in dom', function() {
      element.all(by.repeater('l in myLog')).count().then(function(count) {
        expect(count).toEqual(6);
      });
    });
  </file>
</example>

### Directive Decorator Example

Failed interpolated expressions in `ng-href` attributes can easily go unnoticed. We can decorate `ngHref` to warn us of
those conditions.

<example module="urlDecorator" name="directive-decorator">
  <file name="script.js">
    angular.module('urlDecorator', []).

      controller('Ctrl', ['$scope', function($scope) {
        $scope.id = 3;
        $scope.warnCount = 0; // for testing
      }]).

      config(['$provide', function($provide) {

        // matchExpressions looks for interpolation markup in the directive attribute, extracts the expressions
        // from that markup (if they exist) and returns an array of those expressions
        function matchExpressions(str) {
          var exps = str.match(/{{([^}]+)}}/g);

          // if there isn't any, get out of here
          if (exps === null) return;

          exps = exps.map(function(exp) {
            var prop = exp.match(/[^{}]+/);
            return prop === null ? null : prop[0];
          });

          return exps;
        }

        // remember: directives must be selected by appending 'Directive' to the directive selector
        $provide.decorator('ngHrefDirective', [
          '$delegate',
          '$log',
          '$parse',
          function($delegate, $log, $parse) {

            // store the original link fn
            var originalLinkFn = $delegate[0].link;

            // replace the compile fn
            $delegate[0].compile = function(tElem, tAttr) {

              // store the original exp in the directive attribute for our warning message
              var originalExp = tAttr.ngHref;

              // get the interpolated expressions
              var exps = matchExpressions(originalExp);

              // create and store the getters using $parse
              var getters = exps.map(function(exp) {
                return exp && $parse(exp);
              });

              return function newLinkFn(scope, elem, attr) {
                // fire the originalLinkFn
                originalLinkFn.apply($delegate[0], arguments);

                // observe the directive attr and check the expressions
                attr.$observe('ngHref', function(val) {

                  // if we have getters and getters is an array...
                  if (getters && angular.isArray(getters)) {

                    // loop through the getters and process them
                    angular.forEach(getters, function(g, idx) {

                      // if val is truthy, then the warning won't log
                      var val = angular.isFunction(g) ? g(scope) : true;
                      if (!val) {
                        $log.warn('NgHref Warning: "' + exps[idx] + '" in the expression "' + originalExp +
                          '" is falsy!');

                        scope.warnCount++; // for testing
                      }

                    });

                  }

                });

              };

            };

            // get rid of the old link function since we return a link function in compile
            delete $delegate[0].link;

            // return the $delegate
            return $delegate;

          }

        ]);

      }]);
  </file>

  <file name="index.html">
    <div ng-controller="Ctrl">
      <a ng-href="/products/{{ id }}/view" id="id3">View Product {{ id }}</a>
      - <strong>id === 3</strong>, so no warning<br>
      <a ng-href="/products/{{ id + 5 }}/view" id="id8">View Product {{ id + 5 }}</a>
      - <strong>id + 5 === 8</strong>, so no warning<br>
      <a ng-href="/products/{{ someOtherId }}/view" id="someOtherId">View Product {{ someOtherId }}</a>
      - <strong style="background-color: #ffff00;">someOtherId === undefined</strong>, so warn<br>
      <a ng-href="/products/{{ someOtherId + 5 }}/view" id="someOtherId5">View Product {{ someOtherId + 5 }}</a>
      - <strong>someOtherId + 5 === 5</strong>, so no warning<br>
      <div>Warn Count: {{ warnCount }}</div>
    </div>
  </file>

  <file name="protractor.js" type="protractor">
    it('should warn when an expression in the interpolated value is falsy', function() {
      var id3 = element(by.id('id3'));
      var id8 = element(by.id('id8'));
      var someOther = element(by.id('someOtherId'));
      var someOther5 = element(by.id('someOtherId5'));

      expect(id3.getText()).toEqual('View Product 3');
      expect(id3.getAttribute('href')).toContain('/products/3/view');

      expect(id8.getText()).toEqual('View Product 8');
      expect(id8.getAttribute('href')).toContain('/products/8/view');

      expect(someOther.getText()).toEqual('View Product');
      expect(someOther.getAttribute('href')).toContain('/products//view');

      expect(someOther5.getText()).toEqual('View Product 5');
      expect(someOther5.getAttribute('href')).toContain('/products/5/view');

      expect(element(by.binding('warnCount')).getText()).toEqual('Warn Count: 1');
    });
  </file>
</example>

### Filter Decorator Example

Let's say we have created an app that uses the default format for many of our `Date` filters. Suddenly requirements have
changed (that never happens) and we need all of our default dates to be `'shortDate'` instead of `'mediumDate'`.

<example module="filterDecorator" name="filter-decorator">
  <file name="script.js">
    angular.module('filterDecorator', []).

      controller('Ctrl', ['$scope', function($scope) {
        $scope.genesis = new Date(2010, 0, 5);
        $scope.ngConf = new Date(2016, 4, 4);
      }]).

      config(['$provide', function($provide) {

        $provide.decorator('dateFilter', [
          '$delegate',
          function dateDecorator($delegate) {

            // store the original filter
            var originalFilter = $delegate;

            // return our filter
            return shortDateDefault;

            // shortDateDefault sets the format to shortDate if it is falsy
            function shortDateDefault(date, format, timezone) {
              if (!format) format = 'shortDate';

              // return the result of the original filter
              return originalFilter(date, format, timezone);
            }

          }

        ]);

      }]);
  </file>

  <file name="index.html">
    <div ng-controller="Ctrl">
      <div id="genesis">Initial Commit default to short date: {{ genesis | date }}</div>
      <div>ng-conf 2016 default short date: {{ ngConf | date }}</div>
      <div id="ngConf">ng-conf 2016 with full date format: {{ ngConf | date:'fullDate' }}</div>
    </div>
  </file>

  <file name="protractor.js" type="protractor">
    it('should default date filter to short date format', function() {
      expect(element(by.id('genesis')).getText())
        .toMatch(/Initial Commit default to short date: \d{1,2}\/\d{1,2}\/\d{2}/);
    });

    it('should still allow dates to be formatted', function() {
      expect(element(by.id('ngConf')).getText())
        .toMatch(/ng-conf 2016 with full date format: [A-Za-z]+, [A-Za-z]+ \d{1,2}, \d{4}/);
    });
  </file>
</example>