@ngdoc overview
@name Filters
@description

A filter formats the value of an expression for display to the user. They can be used in view templates,
controllers or services and it is easy to define your own filter.

The underlying API is the {@link api/ng.$filterProvider filterProvider}.

## Using filters in view templates

Filters can be applied to expressions in view templates using the following syntax:

        {{ expression | filter }}

E.g. the markup `{{ 12 | currency }}` formats the number 12 as a currency using the {@link api/ng.filter:currency `currency`}
filter. The resulting value is `$12.00`.

Filters can be applied to the result of another filter. This is called "chaining" and uses
the following syntax:

        {{ expression | filter1 | filter2 | ... }}

Filters may have arguments. The syntax for this is

        {{ expression | filter:argument1:argument2:... }}

E.g. the markup `{{ 1234 | number:2 }}` formats the number 1234 with 2 decimal points using the
{@link api/ng.filter:number `number`} filter. The resulting value is `1,234.00`.


## Using filters in controllers and services

You can also use filters in controllers and services. For this, add a dependency with the name `<filterName>Filter`
to your controller or service. E.g. using the dependency `numberFilter` will inject the number filter.
The injected argument is a function that takes the value to format as first argument and filter parameters
starting with the second argument.

The example below uses the filter called {@link api/ng.filter:filter `filter`}.
This filter reduces arrays into sub arrays based on
conditions. The filter can be applied in the view template with markup like
`{{ctrl.array | filter:'a'}}`, which would do a fulltext search for "a".
However, using a filter in a view template will reevaluate the filter on
every digest, which can be costly if the array is big.

The example below therefore calls the filter directly in the controller.
By this, the controller is able to call the filter only when needed (e.g. when the data is loaded from the backend
or the filter expression is changed).

<doc:example module="FilterInControllerModule">
<doc:source>
<script>
  angular.module('FilterInControllerModule', []).
    controller('FilterController', ['filterFilter', function(filterFilter) {
      this.array = [
        {name: 'Tobias'},
        {name: 'Jeff'},
        {name: 'Brian'},
        {name: 'Igor'},
        {name: 'James'},
        {name: 'Brad'}
      ];
      this.filteredArray = filterFilter(this.array, 'a');
    }]);
</script>

<div ng-controller="FilterController as ctrl">
  <div>
    All entries:
    <span ng-repeat="entry in ctrl.array">{{entry.name}} </span>
  </div>
  <div>
    Entries that contain an "a":
    <span ng-repeat="entry in ctrl.filteredArray">{{entry.name}} </span>
  </div>
</div>
</doc:source>
</doc:example>


## Creating custom filters

Writing your own filter is very easy: just register a new filter factory function with
your module. Internally, this uses the {@link api/ng.$filterProvider `filterProvider`}.
This factory function should return a new filter function which takes the input value
as the first argument. Any filter arguments are passed in as additional arguments to the filter
function.

The following sample filter reverses a text string. In addition, it conditionally makes the
text upper-case.

<doc:example module="MyReverseModule">
<doc:source>
<script>
  angular.module('MyReverseModule', []).
    filter('reverse', function() {
      return function(input, uppercase) {
        var out = "";
        for (var i = 0; i < input.length; i++) {
          out = input.charAt(i) + out;
        }
        // conditional based on optional argument
        if (uppercase) {
          out = out.toUpperCase();
        }
        return out;
      }
    });

  function Ctrl($scope) {
    $scope.greeting = 'hello';
  }
</script>

<div ng-controller="Ctrl">
  <input ng-model="greeting" type="greeting"><br>
  No filter: {{greeting}}<br>
  Reverse: {{greeting|reverse}}<br>
  Reverse + uppercase: {{greeting|reverse:true}}<br>
</div>
</doc:source>
</doc:example>

## Testing custom filters

See the {@link http://docs.angularjs.org/tutorial/step_09#test phonecat tutorial} for an example.
