Why does the AngularJS filter work only once?

Question

Consider the following example:

    angular.module('app', []).controller('TestController', function($scope) {
      $scope.getText = function() {
          console.log('getting text');
          return 'text';
      };
  }).filter('text', function() {
      return function() {
          console.log('text filter');
          return 'text';
      };
  });

 

  <script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.6.2/angular.min.js"></script>
<div ng-app="app" ng-controller="TestController">
    <p>{{getText()}}</p>
    <p>{{'' | text}}</p>
</div>

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Note that the function getText()is executed twice, while the filter only works once. I assume the function getText()runs twice to make sure the model is now stable. Why isn't the same behavior for a filter?

Answer 1

The documentation is pretty clear on this:

In templates, filters are only executed when their inputs have changed . This is more efficient than filtering for each $ digest, as is the case with expressions.

Here is the source.

Answer 2

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---

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angular.module('app', []).controller('TestController', function($scope) {
  $scope.foo = 'bar';
  $scope.getText = function() {
    console.log('getting text');
    $scope.foo += 'a';

    return 'text';
  };
}).filter('text', function() {
  return function() {
    console.log('text filter');
    return 'text';
  };
});

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.6.2/angular.min.js"></script>
<div ng-app="app" ng-controller="TestController">
  <p>{{getText()}}</p>
  <p>{{foo | text}}</p>
</div>

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Answer 3

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Answer 4

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Answer 5

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{{getText() | text}}