Filters are just functions which transform input to an output. However filters need to be Dependency Injected. To achieve this a filter definition consists of a factory function which is annotated with dependencies and is responsible for creating a filter function.
// Filter registration
function MyModule($provide, $filterProvider) {
// create a service to demonstrate injection (not always needed)
$provide.value('greet', function(name){
return 'Hello ' + name + '!';
});
// register a filter factory which uses the
// greet service to demonstrate DI.
$filterProvider.register('greet', function(greet){
// return the filter function which uses the greet service
// to generate salutation
return function(text) {
// filters need to be forgiving so check input validity
return text && greet(text) || text;
};
});
}The filter function is registered with the $injector under the filter name suffix with
Filter.
it('should be the same instance', inject(
function($filterProvider) {
$filterProvider.register('reverse', function(){
return ...;
});
},
function($filter, reverseFilter) {
expect($filter('reverse')).toBe(reverseFilter);
});For more information about how angular filters work, and how to create your own filters, see Filters in the Angular Developer Guide.