Filters in AngularJS can form a pipeline to format and adapt model data inside a view. The built-in filters for AngularJS are date (format a date), filter (select a subset of an array), currency (format a number using a currency symbol for the current locale), and many more. I say filters can form a pipeline because you place them inside expressions in a view using a pipe symbol. The canonical example of a filter is the filter filter, which in the following markup filters a list of items according to what the user types in the search input (try it live here).

<input type="search" placeholder="Filter" ng-model="searchTerm"/>    

<ul ng-repeat="item in items | filter:searchTerm | limitTo:maxItems">
    <li>{{item}}</li>
</ul>

Notice the output of filter is also piped to a limitTo filter, which restricts the number of items in the list to maxItems (where maxItems would need to be defined in the model).

Custom Filters

Custom filters only require a filter function to be registered with Angular. The filter function returns a function that will perform the actual work. For example, a plurify filter to add the letter s to the output:

(function() {

    var plurify = function() {
        return function(value) {
            return value + "s";
        };
    };

    angular.module("patientApp.Filters")
           .filter("plurify", plurify);
}());

Now plurify is available to use in a view:

<ul ng-repeat="item in items | filter:searchTerm | limitTo:maxItems">
    <li>{{item | plurify }}</li>
</ul>

A filter can also take one or more optional parameters.

var plurify = function() {
    return function(value, strength) {          
       strength = strength || 1;
       return value + Array(strength + 1).join("s");
    };
};

Now we can add 5 s characters to the output:

<li>{{item | plurify:5 }}</li>

The beauty of filters is how you can package up small pieces of testable code to make the job of the model and view a bit easier, and the syntax to use a filter is easy and intuitive.